What's wrong with me? Multiple personality disorder?!

Most likely, it's more hormonal than anything else. However, it does sound a little bit like bi-polar disorder. I'm not insulting you, but I chuckled when I saw the multiple personality disorder question because there is a large school of people who really don't think multiple personality disorder exists. I chuckled, but was mad at the same time because this is what TV does, in my opinion. People, especially people who are currently in their teens or early 20s don't really understand what mental illness is. I'm not saying that I don't think multiple personality disorder exists. Also, it's now called Dissociative Identity Disorder and it's when people can't account for a large period of time and it's like other people are trapped in your body. This disorder USUALLY is experienced by people who have suffered significant abuse, whether it be sexual, physical, or emotional, and any combination of all three. Usually people with DID start to show symptoms at a relatively young age. The different personalities seem to take on roles, such as a male personality who is the protector, and a 6 year old who is still the innocent child. It's very complicated. However, I do think it's rare. That's why I mentioned how mental illness is portrayed on TV. They have it wrong, which adds to the stigma and just makes it difficult for people to seek help.
My suggestions are the following: 1. keep a journal. Rate the way you are feeling daily, one being suicidal with a plan (by the way, if you feel suicidal you should seek help immediately), 10 being super happy, high as the clouds. Include things like going on shopping sprees without thinking about how much money you have in your bank account and feelings like you could jump out of your skin, and, of course, everything else in between. Make sure you note if you are on your "cycle", if it's several days before your cycle starts, and several days after your cycle ended. Also include pertinent information like having a fight with a sibling, parent, or a friend (boy or girls). This may help you recognize a pattern. You may also find that writing out your feelings help regulate your emotions. If it does, that's wonderful.
2. See your regular doctor and make sure that these mood swings aren't hormonal, and this time I don't mean from your "cycle". You may need to have your thyroid checked to make sure it's not under active (or over active, for that matter). It's not a big deal, it's just a blood test. Your thyroid plays a very important role and if it is out of whack, it could be contributing to your moods.
3. If this continues, you may want to think about seeing a psychiatrist, even to rule out an issue like bi-polar. I'm not saying that it is, but it isn't anything you would want to ignore either.
I hope you feel better!
****Any advice you receive on Yahoo Answers should not be substituted for a medical exam and medical advice.

personal experience, signifcant family history of mental illness, surviving two brothers' suicides

that's totally normal for a teenager, it's a hormonal thing, but if you do think that it might be something else you could always see a professional

im Pretty sure its just hormones because if it was multi-personality you wouldn't be able to tell what time it is when you change you would be constantly changing throughout the day

Pregnant? That's how my wife and I found out; she WAS throwing tantrums

You did not state how old you were.

If you are going through puberty, it is possible that this is what is causing you to have such opposite expressions of mood. If this is the case, you need to make an appointment with your doctor so that he/she will be able to treat this in a way which will be conducive to that type of medical problem.

If you have gone through puberty, it is then possible that you are experiencing a mood disorder and need to be obtain help from a mental health professional and treatment with a medication.

It is not uncommon for someone with a mood disorder to vacillate from one end of the spectrum of mood (being extremely down to that of being extremely high) to the other.

If this is the case, you need to obtain this treatment as soon as possible. The longer you wait, the more difficult it will be to treat and the longer it will take to treat.

Multiple personality disorder is a whole other animal - like you are sometimes a 10 year old boy and a 80 year old great-grandmother.

What you are describing can be a part of the hormonal problems that fluctuate quite drastically in some teen girls and even adult women. You might want to start a journal about when these mood swings affect you and at what time of the month they come on. If you see a pattern that develops over the course of your menstrual cycle, you might be able to pinpoint your good days and bad days and take steps to help yourself - such as not trying to schedule too much during your down days, limiting caffeine, getting extra rest, eat more healthy foods, etc. If you find that the small things really don't help, then you might want to confide in your mom and dad or a trusted adult who can help you figure out what's wrong. Try to create a timeline, also, to see if you can figure out when this started and what the circumstances were at the time, and if anything new came into your life then. Sometimes that can give you more insight, as well.

And what Amin K. posted is excellent advice.

Personal experience.