If you are concerned that you may be pregnant or are worried about having contracted a sexually transmitted disease, please call your area Planned Parenthood office - their number is in the phone book. They will counsel you and help you get any tests you require. I guarantee you that they will treat you with respect and compassion. I'd also like you to consider talking to your parent(s) about what happened. I know that you believe they will respond in negative and irrational ways. I know that you might feel shamed if you told them, thinking they would lose all respect for you. But they were teenagers too, tempted by the same things that caused you to make this mistake. If you could summon up enough courage, you could give them a chance to understand and to be supportive.. I know that's asking a lot of you but you may be very pleasantly surprised at how they will appreciate your courage and your confiding in them.
If you feel more comfortable talking with just your mom, maybe reading "Venus in Blue Jeans: Why Mothers and Daughters Need to Talk About Sex", by Nathalie Bartle, before you talk to her will help you discuss this incident and other sexual matters with her.
Please make the call that I suggested and get some counseling and support. There are more kids your age postponing sex than you might imagine. I know that you want it to be much more than what you experienced. Forgive yourself, get some, help and think about reading that book and talking with your mom. Thanks for writing.