This is the very reason I videotape all sexual encounters...
On a serious note...I agree that talking to a lawyer would be a prudent decision. I would also save ANY and ALL correspondance between you whether email or written. Of course talk to the friend with the condom and get their story. This would be very hard to prove especially with no physical evidence due to the timeline you described.
I am surprised that she was harsh to you in her email. Did she mention rape at all, or is she just pissed because you never called her back?