I'm an IV meth user. Sometimes when addicts do a shot they "get wet" or "come in their pants". I'm neVER supposed to have my period, but sometimes do a shot that makes me bleed.. a lot. Why?

Please get help. Please get help for this addiction, it will ruin your life. If you are never supposed to have a period and you are bleeding then there is something wrong and you need to see a doctor and get a pelvic exam to determine the cause. I hope will see a health care provider as soon as possible.
Ischemia. Meth cuts off the blood supply to your uterus, and the inside lining dies off and is shed. Hence the bleeding. This is the same reason that "meth" causes brain damage in unborn children. I agree with my colleagues. Adults have lied to you about other things being very bad, but the dangers of meth are real. The meth-heads I've autopsied HAVE torn the flesh from their bodies. Don't die like that.
I'm so sorry. It really is upsetting and saddening that the question you are asking is so trivial compared to what is really important to your health. The health effects of methamphetamine are so life-threatening, especially to your nervous system, that I suspect no one has studied what it might do to hormones and menstrual or vaginal bleeding! It doesn't matter why...just stop using or get help to stop. PLEASE.