For girls only! PLEASE HELP:( I'm scared?

Hey, I'm 18 but I think I might have a chemical/hormone imbalance. My period is usually very regular but I have have not had my period for about 2 months now. I do not have , but I do "mess around" with my boyfriend. I have taken about 5 pregnancy tests just because I have been freaking out and they have all been negatives. I have been really bloated and tired lately. I have been playing tennis a lot lately, but I don't get why I would be bloated if that was the reason. I am just scared and don't know why I haven't gotten it! Please help me:( Could it be an iron dificiency? And if all tests said negative, is there still a possibility that I am pregnant? Even though I have never had ?....PLEASE PLEASE PLEASE help:(( I'm stressing and freaking out abut this. It's all I think about:(