"I have a healthy fear of the unknown and the unknown is if he is a murderer or not," McClain, now Chapman, tells PEOPLE

Asia McClain was a classmate of 17-year-old Adnan Syed’s at Woodlawn High School in Baltimore when he was charged with murdering one of their classmates – Syed’s ex-girlfriend, Hae Min Lee.

McClain, who now goes by her married name, Chapman, says she was in the library with Syed for about 15 to 20 minutes after school on January 13, 1999, the day prosecutors say Lee was murdered.

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Soon after Syed’s arrest, Chapman wrote two letters to him in jail, saying she would be his alibi for that time period – during which prosecutors later alleged Syed killed Lee.

But Syed’s original defense team never contacted Chapman, and for a long time, she believed she had no relevance to Syed’s case, which resulted in a guilty verdict and a sentence of life in prison.

That all changed when Chapman listened to Serial, a popular podcast downloaded by millions and which raises questions about Syed’s case, highlighting Chapman’s role as a key alibi witness.

Syed has always proclaimed his innocence. This past February, after years of unsuccessful efforts to seek a new trial, a Maryland judge presided over a post-conviction relief hearing where new evidence was presented – including the riveting testimony of Chapman.

Former prosecutor Dave Irwin told the court that had Chapman been called to testify during Syed’s 2000 trial, her testimony could have changed the results of the proceedings.

As retired judge Martin Welch continues to decide whether or not to grant Syed a new trial, Chapman, 34 – now a Spokane, Washington stay-at-home mom of two with another child due June 29 – has written a revealing book about her experiences with Syed and the case.

Confessions of a Serial Alibi is set for release June 7.

Chapman tells PEOPLE exclusively that she wrote the memoir to “relieve the pent up anxiety and emotions” she’s felt since Serial, a spinoff of This American Life, was first broadcast in 2014.

Unwanted Notoriety and Stress

Serial producer Sarah Koenig had interviewed Chapman for the podcast, but Chapman admits she had no idea that granting the interview would lead to such widespread notoriety and worldwide scrutiny.

Upon first clicking on the podcast, she writes: “…there in big bold letters I saw the words “The Alibi” and I was mortified…By the time I finished the first episode I was pacing around the house having a full-fledged fit.”

That was just the start of her distress. The podcast gave birth to armchair detectives and conspiracy theorists who created a lot of “misperceptions” about Chapman, she tells PEOPLE.

Chapman’s ulcerative colitis, which had been in remission, returned, “triggered by stress,” she says. “My body began producing ulcers. I lost two children due to miscarriage” from issues that go along with the condition. (Since completing the book, she says it’s back in remission.)

Unsure if Syed is Guilty or Innocent

The book details Chapman’s journey: from high school memories of both Syed and Lee, to her chance encounter with Syed in the local library during the time prosecutors say Syed murdered Lee, to her riveting testimony this past February.

A significant basis of Syed’s recent bid for a new trial rests on the mistake he says his former attorney, Cristina Gutierrez, made failing to contact Chapman to investigate her as an alibi witness for his 2000 trial.

Chapman tells PEOPLE she doesn’t know if Syed is “innocent or guilty.” She tells PEOPLE: “A retrial is a good thing.”

“If there is a chance Adnan is innocent, we as a society – meaning the criminal justice system – we owe it to them to figure out if he is guilty or innocent,” she says.

In February, Lee’s family issued a statement denouncing Chapman’s testimony. “Whatever her personal motives, we forgive her,” they said, “but we hope she will not use Hae’s name in public, which hurts us when we hear it from her.”

Chapman feels badly for the Lees. “I know that Hae’s family is hurting and my heart goes out to them concerning her death,” she tells PEOPLE. But Chapman believes she had to do the right thing and testify.

“What if it was Hae was in prison proclaiming her innocence?” she says. “Would they not have me testify?

“If I was in prison wasting away on charges I proclaim to be false, and there was a person who knew that they saw me during the time I was supposed to be committing the crime,” she says, “I would hope to God that person would have the fortitude to come forward and say, ‘I was was actually with this person.'”

Recurring Nightmares

During her February testimony, Chapman appeared poised and unflappable, even during a withering cross examination by the state. Internally, however, things were quite different.

“You have a lot of adrenaline pumping through you, you are scared, you’re nervous,” she says. “You don’t know what questions are going to come at you.”

What was it like to see Syed, sitting just feet from her in the courtroom, after all these years? She takes a few seconds to respond. “The best word to describe it is odd,” Chapman says.

Chapman tells PEOPLE she doesn’t believe Syed ever looked at her during her two days on the stand.

She writes she has recurring nightmares about a particular imaginary scenario: Syed is retried and found not guilty, but subsequently, he confesses to Chapman than he actually committed the murder and then begins to strangle her – the same way he allegedly strangled Lee.

“It’s at these moments in the nightmare that I usually wake up in a cold sweat.”