Don't think of yourself in categories of a bad person.
If you think it was a mistake, make sure you learn from it, so next time some random guy doesn't just "end up on top of you". And try to be more... aware of things (things happening down there).
First things first - just to put your mind at ease get a pregnancy test and do see. Make sure you follow the instructions, etc. You might want to repeat it after a week or so to be double sure.
It's been a while since you posted, so you might already be ok

but if not - I believe the "symptoms" are caused by your mind - by how you feel about what happened and how scared you are. You should be fine (I'm basing it on the details you gave us).
Take care of yourself!