Not to get too personal but, was this your first time? Because it's normal to have some amount of fear, maybe a lot, in that circumstance. Sometimes people can get shocked back into the realization that THERE IS ANOTHER PERSON ALL OVER YOU. Humans are very large animals, larger than just about everything that ever lived, and having one all up in your personal space can be startling if you're not used to it.
Also, as I'm sure you're aware, many people on the autism spectrum are highly touch-sensitive and easily agitated. Plus the alien factors of not knowing her so well, getting drunk / high for the first time.... it can leave you with more negative emotions than positive ones.
You were a consenting adult, but you now regret it. Those are not antonyms. It is entirely possible to regret, and have legitimately awful feelings about, a consensual and undamaging sexual encounter.
Look at it this way: you paid your entry fee for the roller coaster, and your head whipped around too hard and you got bruised up by it. The roller coaster operator wasn't aiming to hurt you and you are in no way benefited by thinking he did. You are still allowed to feel bad about it.
"Don't think it hasn't been a little slice of Heaven just because it hasn't!" --Bugs Bunny