EXCLUSIVE

"Evel Dick" Donato says a romp with a Vegas stripper is why he's HIV positive ... so he thinks.

Dick tells TMZ ... he had unprotected sex with the stripper -- who he says got her period during intercourse. Dick says, "When I woke up, she was gone and I looked like I had murdered someone."

Donato was diagnosed in 2011 ... from blood tests administered on "Big Brother." Donato says he left the show and broke the news to his mother and his girlfriend ... calling it the "hardest conversation I have ever had to have."

Dick says his GF -- who tested negative -- was amazing ... assuring him they would get through it. She stuck by him for the next two years, until they split in 2013. They're appearing on "Couples Therapy" to see if they can re-ignite their relationship.