Okay, I'm teenage girl of 15 yrs. I think of myself as a very creative and outgoing person, and I'm often described as "too nice". I'm very active, smiley, and enjoy being around other people, and helping them. I'm in a good mood most of the time, but when I'm not, there's something extremely wrong. When I get stressed, extremely excited, or scared, my body shakes and I can't stop it, sometimes I cry and have trouble breathing. When I get disappointed it seems as if my whole deminor changes, I cry often, and have homicidal, ore suicidal thoughts.. then in a day or two, I'm back to my excited, smiley self. Is there something wrong with me, or am I imagining things?

Most likely, it is just the stress level causing you to lose your optimistic personality and pushes you to something that is completely opposite. It may just be those few days you need to cope with stress by being someone who isn't as happy.
Worse case scenario is that you have a form of bipolar disorder. It may be something to talk to your doctor about, to get a professional opinion

This could be anything from simple stress, to bipolar or boarderline personality disorder (both of which I have personally lol and they suck)

I say your best course of action is to ask your parents to take you to your doctor and talk to him/her about it, possibly get a referral to a psychiatrist if your doctor can't find a phsyical reason for these issues, as there are many physical things that can cause this as well, hormonal imbalances, polycystic ovarian syndrome, thyroid issues, etc.

Good luck hon!