This is not uncommon, the feeling of guilt and being unnatural around people you would normally feel comfortable around. Do you go to school? If so, the school probably offers free counseling and I urge you to take advantage of the service. When you go to the counselor be sure to be completely 100% honest with them so they can help work out this issue.
If you do not go to school, I would suggest that it is imparitive to see a counselor or psychologist about your feelings on this issue. Medications will NOT work and in fact it will only create more side affects that will make you unhappy and feel unnatural.
The counselor/Psychologist ( You can even find cheap ones at universities that have Graduate students that are certified but gaining practicing hours) will help you talk about the event, your feelings and things you associate with what happened in order to confont it and change your pattern of thinking. I know it may be difficult to see now, but you are not guilty of anything and if you don't talk to someone about it now, it will haunt you for a very long time.
I hope it works out for you.