Maddielou,
If the negative tests where properly done, far enough after the date of possible conception, then the chances of you being pregnant are actually very small.
However, hormonal imbalances are common in young women. These should be evaluated and treated. You may need to be placed on birth control pills to regulate your cycles.
You really should see a gynecologist. Some family practitioners also do adolescent gynecology. But, since you are sexually active, you should be seeing a gynecologist at least yearly.
The symptoms you are describing can also be coming from the irregular cycles and also from stress. But, you do need to find out if you are pregnant or not. It is very doubtful that you are, but the gynecologist can determine that once and for all.
Again, you really need to be seeing a gynecologist on a regular basis and you need to get your cycles regulated. I'm not going to go into the you should have been more careful, but...you know condoms are easily obtained.
Good luck.