Firstly, do your friends know how worried you are? I know boys will be boys but if they do then they're not being particularly helpful.
secondly, you say you used a condom - the chances of pregnancy would be very very low. The chances of any kinds of diseases would be too, but to put your mind at rest, as the other AB people advise, get checked out.
You are single and she told you that she was single. So you're two consenting adults who have done nothing wrong. If you had started promising her a relationship etc, then that would have been unfair.
It sounds like it's got a bit out of hand and I don't think your friends are helping with their rumours etc. I think I would learn from the experience and keep your head down for a week or so and see if it dies down. Friends aside, have you heard anything from anyone else? If not, then perhaps they are pulling your leg. Is one friend a little more sensitive than the others? Perhaps you could phone him when he is away from them and say it's really worrying you so if they are joking please could he let you know kind of thing.