My girlfriend is freaking out because we had sex for the first time and she thinks she might be pregnant. We used condoms every time and I pulled out before ejaculating every time. She was supposed to start her period today but just had some very very light red spotting when she wiped after urinating. Last week she had fairly heavy spotting for 6 days where she was passing clots and it was a dark brown color. She says it feels like her ovaries are moving and has a sharp pain around her belly button. She is really stressing out and calls me every day asking me to look up what each symptom means and I can't find every answer so I'm hoping someone knowledgeable can help me here. She also has headaches, mild backaches, and mild nausea. She is also a gymnast and a cheerleader and she thinks the backaches could be from the gymnastics she's been doing the last three days.
Any help you could give would be immensely appreciated.

The worry itself can be enough to throw off a cycle for some people.

She may just be getting ready to start her period, that can cause some of the symptoms described. Periods can be off once in awhile, especially if she is in her teens.

Have her wait another week and see what happens with her usual period. If it still doesn't show, she can try a pregnancy test. If it is negative, she can wait until the next month and test again if still no period. The bleeding she had the previous week, may have been an off period already. If she has been very active lately with gymnastics and cheerleading, that too may be enough to throw off her cycle. Usually has to be quite strenuous for a period, but people respond differently.

If you are going to continue having sex, she may want to also consider another birth control option along with using condoms to lessen the odds even more and perhaps give her some greater peace of mind. Ideally you will hold off on the sex until you both are in a better position to handle any unforseen consequences.

Just remember, pregnancy is always possible as no form of birth control will be 100% effective 100% of the time even when used correctly.