A pregnancy test should have been accurate at the time that you took it (assuming that you followed the directions), but it definitely doesn't hurt to do another to double check.
As for getting sick and having diarrhea, it could be a stomach bug, or the flu, or something like that.
The missed period/spotting could be due to your birth control. Birth control pills give many women lighter periods, some pills can make you only have four periods a year, some women have spotting between periods, etc.
Or both the feeling sick and the missing/light period issue could be due to stress and worrying about pregnancy. In fact, stress is a very common cause of a missed period.
But the only way to find out for sure what's going on would be to talk to a doctor about it. If you need to see a doctor for anything related to reproduction (like getting birth control or different birth control, getting STD tests, getting a pregnancy test, discussing abortion, etc.) you can go to a clinic like Family Planning or Planned Parenthood (I believe Planned Parenthood does abortions. Family Planning does not, but they can still give you information and refer to a doctor who does do them). You can check
http://www.plannedparenthood.org/ to look for a clinic near you.