It was a cold March day as I walked to work from my Hell’s Kitchen studio. The weather was clear, people were out in their coats and scarves, but something did not feel right. The sky was so blue, it hurt my eyes. The billboards in Times Square assaulted me with violent reds, yellows and purples. It was like the world had become brighter, louder, more painful.

When I got to the Midtown newsroom of the New York Post, where I was a year into my first full-time reporting job, I asked a friend, “Have you ever not felt like yourself? Have you ever felt completely off?”

“Sure,” she laughed. “Of course. I hardly ever feel like myself.”

I laughed, too, but inside I worried. Lately, there had been more and more days like this. I had started to feel numb on the left side of my body. My arm and leg on that side tingled. Until recently, I had been a healthy 24-year-old, happily starting a career in the Big Apple with a new boyfriend. Now I spent sleepless hours wondering what was happening to my body and my mind.

Then, one Friday at my desk, I cracked.

I had just finished an interview, was starting to write, and for no reason I began crying hysterically. Co-workers gathered, wondering what was wrong. Moments later, I brushed it all off, and skipped down an office hall, giddy with excitement.

Later, at home, I panicked. Was I going mad? Was I mentally ill?

I contacted a neurologist. He sent me to a litany of tests, including two MRIs. Nothing.

“Maybe it’s mono,” he suggested. “Or a virus.”

He promised to follow up with more tests. In the meantime, I curled up with my boyfriend to watch the PBS show “Spain — On the Road Again.” I remember thinking how much I disliked Gwyneth Paltrow. Then, he would tell me later, I started grinding my teeth, moaning and biting my tongue, until I finally passed out.

It was my first seizure. And the last thing I would remember for a month.

Through interviews with family, my boyfriend and friends, I’m able to cobble together the darkest moments of my life. My father kept notes in a spiral notebook, details of what was happening that he could pass along to my mom as they took shifts caring for me.

From my boyfriend, I know that I awoke after the seizure at St. Luke’s Hospital next to a puking man.

Doctors ordered a CAT scan that came up clean — but my behavior was becoming increasingly bizarre.

“I’m dying of melanoma,” I told my boyfriend. (I had been diagnosed with the skin cancer five years ago and was convinced that was the cause of my seizure.) I started ranting about suing the doctors and believed that they were out to get me.

But even after the seizure and the paranoid delusions, the neurologist didn’t seem worried.

“It’s probably alcohol withdrawal,” he told my stunned mother and me. He was convinced I was an alcoholic; I barely drank.

My family packed up my clothes and took me to my mother’s home in New Jersey, where I continued to deteriorate.

I stopped sleeping. Refused to eat. Paced the halls. Couldn’t control my rambling thoughts, and ranted to my mother. I convinced myself that I was bipolar and that I was having a nervous breakdown.

Then I had a second seizure. Blood and foam spurted out of my mouth onto the Oriental rug in my family room.

Overwhelmed by my erratic behavior, my mom dropped me off at my dad’s house in Brooklyn Heights for one night before my doctor’s appointment. My paranoid delusions got worse. I refused to sleep and started banging on the locked door, screaming, “Get me out of here!” I imagined that my father had murdered his wife.

The next day, my mother, stepfather and boyfriend took me back to the neurologist for an EEG, which records the electrical activity of the brain. On the ride there, I opened up the car door and tried to jump out. My boyfriend held me down and my stepfather child-locked the car. I screamed hysterically until we got to the doctor.

Horrified by my increasing paranoia, and despite my neurologist’s hesitations, my mom and stepfather took me immediately to the hospital. They demanded that I not be placed in a psych ward.

There I had my third grand mal seizure while waiting for a hospital bed, and was whisked up to the epilepsy floor of New York University’s Medical Center.

For the first three days, I shared a room with three other people, mostly suffering from epileptic seizures.

Technicians glued EEG wires to my head, which snaked into a child’s pink backpack on the side of my bed. I was monitored 24/7 by two video cameras mounted on the ceiling.

Two times, I successfully ripped the electrodes off my head, tugged at my IV as blood erupted from my veins, and ran shrieking down the halls, trying to find an exit. Nurses tackled me and stuck me with a sedative. An imposing Jamaican nurse even sat watch with me for 24 hours so that I couldn’t escape again.

I had to wear an orange wristband that said “flight risk.”

“The physical discomfort of not being able to shower or wash her hair coupled with the fact that she was tethered electronically to a monitor started to take its toll,” my dad wrote in his notebook. “She told me she was dying a little bit every day. I told her to trust me and I would get her out, but we had to find out what was wrong with her. She said, no — get me out now.”

It’s hard for me to hear about the things that I believed during my madness. When I turned on the TV, I imagined I was on the news and that satellite trucks from all the major networks were camped out outside of my hospital room waiting to interview me.

I imagined the woman beside me was tape-recording my conversations and talking about me in Spanish to her family. I was convinced that I could speak Spanish and interpret her words.

Multiple times a day, residents, doctors and nurses would ask me: “What day is it? Where are you? What’s your name?” Though it would take me over a minute to answer, I did so correctly, albeit a little pissed off.

One night a nurse came by to do my vitals and ask the inane questions in the middle of the night. I had finally fallen asleep and was less than pleased. As she bent down to take my blood pressure — which was spiking due to the illness — I wound up and slapped her across her chest. She gasped in disbelief and fear.

Two weeks pass. Every day a new doctor comes to visit. There are internists, infec tious-disease doctors, immune-system specialists, psychiatrists, psychopharmacologists. They send me to MRIs, sonograms, X-rays, CAT scans, PET scans, multitudes of blood tests. All come back negative. No one could give my parents an answer.

My dad came in the mornings, sat with me and fed me breakfast. My mom would come in the afternoons, and my boyfriend came in the evenings and stayed with me until I fell asleep. They never missed a day.

Two comforts for me were apples — which I inexplicably craved, eating three a day — and my boyfriend’s iPod, on which he made a mix of mostly Ryan Adams tunes, which helped me sleep through the night.

My father prayed. “I would walk across town on 33rd Street to catch the subway on Park Avenue South. There is a chapel between 1st and 2nd Avenue that is dedicated to Jesus and Mary. Each time I walked by I begged God to help Susannah,” he wrote. “I even tried to make a deal. I asked God to take me right there on the spot or do anything he wanted with me if only Susannah could be helped.”

The paranoid delusions started to wane as the medications, anti-anxiety drugs Geodon and Ativan, increased. But I was losing the battle. I could no longer read. My tongue curled, making it hard to speak. I had difficulty swallowing. It took me a whole minute to answer the simplest questions.

Day after day passed with no answer. I became more withdrawn. I found it hard to walk with my father and mother around the hospital floor. Close friends were alarmed at my passive and incoherent state. Doctors secretly feared that they were losing me.

The head doctors started speaking of a mythical miracle man named Dr. Souhel Najjar. “He would find out what it is,” they would say. They even nicknamed him “Dr. House.”

My mom requested that we meet this Najjar in person. And after two spinal taps came back with high white blood cell counts — an abnormal occurrence that signifies brain swelling — the case was officially passed on to Najjar, a Syrian-born neurologist, neuro-pathologist and epileptologist at NYU Medical Center.

When he came to speak with us, my family was buoyed by his confidence.

He grasped my hands and said, “I’m going to find out what this is and fix it.”

Then he handed me a pencil and a piece of paper.

“Draw a clock,” he said.

I grasped the pencil and made a circle. Feeling a little confused and put on the spot, I drew in the clock face.

My mother and father gasped.

All the numbers were written on the right side of the clock face, and no numbers were on the left side.

NAJJAR now had five clues as to what was overtaking me: the seizures, the catato nia, the high blood pressure, the high white blood cell count in my spinal taps and the bizarre clock. It was preliminary proof that the right side of my brain (which controls the left side of the body) was inflamed. He believed it was some sort of autoimmune encephalitis, or the swelling of the brain caused by an attack by rogue antibodies.

He decided to send my blood and spinal fluid to a well-respected neuro-oncologist from the University of Pennsylvania named Dr. Josep Dalmau to test to see if rare antibodies were present.

But we had to wait two weeks for the results.

In the meantime, a brain biopsy would be necessary, Najjar said. They would need to cut out a piece of my temporal lobe, my horrified parents learned.

“I can remember being alone in her room when she began to cry,” my dad writes in his diary. “I went over to her on the bed and hugged her when I began to cry. Next thing I knew, Susannah was laughing. I asked why. She told me it was the first time she ever saw me cry and I must have looked pathetic.”

The next day, I was taken to ICU for surgery. I remember opening my eyes as they sliced into my brain and saying something like, “I’m awake.”

The anesthesiologist looked at me and said, “Oh, s- – -.”

My mom thinks it was just another hallucination. It seemed so real to me, but I couldn’t trust my own mind.

After the surgery, the blood work and spinal fluid came back positive for rare antibodies called anti-N-methyl-D-aspartic acid receptor, or anti-NMDAR encephalitis. The name signifies that the receptors in the frontal lobe, responsible for cognitive reasoning, and the limbic system, or the emotional center of the brain, are under assault by the immune system. My body was attacking my brain.

Penn’s Dr. Dalmau had discovered these antibodies in 2003. Until then, people suffering from my madness were misdiagnosed, likely ending up in mental hospitals, if not dead. Experts aren’t sure what causes it, though they believe it’s genetic, not environmental.

According to his studies, the median age for the disease is 20. The youngest is 21 months. About 75 percent of those affected by it are women. All show forms of psychotic behavior, some show signs of catatonia. About 80 percent of patients have seizures and 70 percent of patients see psychiatrists before any other doctors, according to Najjar.

Najjar estimates that nearly 90 percent of those suffering from autoimmune encephalitis go undiagnosed.

“It’s a death sentence when you’re still alive,” Najjar told me. “Many are wasting away in a psych ward or a nursing home.”

I was the first person in NYU Medical Center’s history to be diagnosed with NMDAR encephalitis.

Najjar wasted no time. He immediately hooked me up to an IV and started the treatments.

First came the intravenous immune globulin, which reduces inflammation of the brain. Then came high levels of steroids, which also reduce inflammation. And finally he hooked me up to a plasmapheresis machine, which flushes out the harmful antibodies from your system, through an IV in your neck.

The initial treatment took about a week, until I was well enough to be released from the hospital. Six months later, I’m still taking steroids, but I’m back at work, back at home, seeing colors clearly and not breaking down at my desk.

At a medical conference last week at NYU, Najjar presented my case and the wide spectrum of autoimmune encephalitis disorders, saying that I was back to “normal.” A friend and co-worker laughed. “You’re better, but you’re not normal,” she said.

My father reflected on my time in the hospital. “Najjar told me she could have easily ended up in a nursing home for the rest of her life. Najjar thought she was extremely lucky. He was saying, with time, she could get back 90 percent of her cognitive abilities,” he wrote.

I’m happy to say that today I’m at 100 percent, and marveling at the lost month of my life, paging through my father’s diary like I’m reading about a stranger.

“All I knew was that she was alive, and her spirit was intact,” he wrote at the end, words that bring me to tears. “We had more hospital stays for treatments, doctor visits, and lots of medications to deal with, but my baby was on the way home.”