Okay, so I have just joined my school swim team and we have been doing very hard workouts that include the stomach area. I also had sex with my boyfriend 5 days ago and my stomach hurts and I'm always tied. Im only 15 and do not want to be pregnant or even have a baby at this age. Im always afrad to ask my mom for birth control. Im even more afraid of being pregnant... I don't know how to tell my mom or even talk to my mom about sex related things.

If you are going to make the mature decision to have sex, you will need to make the mature decision to talk to your mom about birth control. If nothing else, your partner should be using a condom... and using it correctly.
However, even with birth control, pregnancy is not always prevented... it only lowers the risk. People do get pregnant while using birth control.

No one thinks it will happen to them, that it always happens to someone else. Well just remember, to other people... you are someone else.

If you aren't comfortable bringing it up to your mom, maybe write her note and let her know you want to talk to her.

Since having a baby at this time would really mess things up for you, why not make the decision to hold off on sex altogether? Then you will have absolutely nothing to worry about and you can really enjoy the swim team, school activities, your friends, etc. without that concern.

Just as a side note... if you decide having sex right now isn't the best idea for you and your boyfriend tries to persuade you otherwise... dump the boyfriend. He should also be wanting what is best for you and the chance of a baby at 15 isn't it.