Most American kids in their early teens are crazy almost by definition. But you have isolated your case and I think that's important because it suggests that there might be a cause outside yourself, or things that you cannot fix by yourself (as opposed to matters of self-respect, such as making a point of being extra-polite or tolerant). You also ruled out PMS, which was the logical thing to do.
No one is calling you mentally unstable, but you owe it to yourself to talk with your family doctor (if you don't have one, get one or, in a pinch, a pediatrician). Tell him/her EVERYTHING that bothers you, even stuff you might think is trivial, and answer any questions as honestly as you can (there won't be trick questions, I can assure you). If the doc. refers you to a counselor or therapist, go. And if you don't mind my saying so it is good of you to work on this now, rather than push it forward into your senior high or college years -- some kids do that and all Hell breaks loose. You are doing the right thing by facing any such issues early.
Good luck!
