My first thought was could you have possibly have been slipped a drug of some kind? Sounds very scary indeed
I agree with getting all the facts before you tell your boyfriend anything. Obviously if you have NO recollection of what happened then you need to gather as many facts as you can and definitely get yourself checked out, you may even need the morning after pill (not sure what it's called outside of the UK, but it can prevent pregnancy if taken up to 72 hours after unprotected sex) There may have been more happened than any of you know about.
How awful for you. At the moment though concentrate on getting the facts and getting yourself looked at. Take care.