Hi,
Anytime that you think you might have missed your period because of a pregnancy risk, you should take a pregnancy test(s) for an accurate answer. You can either take home pregnancy tests (hpt's), or go to a local health clinic to be tested. If you haven't had sexual intercourse (sex), or haven't engaged in foreplay that involved ejaculation (release of semen from a male) then you don't need to be worried about pregnancy. Fingering doesn't cause pregnancy...
You may be experiencing irregular periods, or a skipped period, which could be the reason why you haven't had a period yet. Missing one period can happen to some women from time to time. There are all kinds of reasons for stomach pains, not just pregnancy. The lower pain you've had might be due to menstrual cramps, an illness, or from some other health issue. If you continue to have a lot of discomfort, you really should speak with your foster mom, or an adult you can trust, so that you can get treatment for the pain. I hope you feel better soon :)