Pregnancy tests aren't going to show anything unless you've been pregnant for at least 2 weeks. (I believe that's the number - look it up). But if you've had a pregnancy test within the last month and it is still negative, nickie is probably right and you should start the birth control again to regulate your cycles.
If you're still worried, ask your primary care physician to refer you to a good ob/gyn. You should be making yearly visits anyway, especially if you are sexually active. Also, if you're worried about getting pregnant any time in the future, it's really important to be on vitamins (ie folic acid) to prevent some nasty birth defects. We're told over and over again in school to recommend folic acid supplements for anyone your age who is sexually active. Good luck!
I think that your symptoms might be close to pregnancy symptoms because when your hormones get out of whack then they can mimic pregnancy and PMS hormones. I think that because you got off of your birth control pills the bleeding you were probably experiencing break through bleeding (which happens a lot when taking a new birth control or stopping it suddenly) and now you are not having a period because you are off of your birth control which helps regulate your periods. If you blood tests are coming up negative then you were probably not pregnant and aren't pregnant. I think you could definitely go back onto the pills when you get your next period without worry. Hope you feel better soon =)