I don't think it's normal, but at 15 it might be, since your hormones are still just getting into gear. Did you keep any kind of chart when you were 11 (or 12 or 13 or 14) that clocks your menstrual months, so you could show a doctor that at a different age your cycles were longer? It almost sounds like you have a fibroid, but that would be unusual at age 15 too.
I would chart this all out on a calendar and go see the doctor. He or she should be able to do some hormone tests and an ultrasound, at the least, to see if any obvious answer pops up.
In the meantime, take iron (with vitamin C to help absorbtion and a fiber gummy to help the iron not constipate you) and also take Advil or Nuprin for the cramps. They are anti-prostaglandins and do really help.
Hi. Gosh this is a lot, sweetie. It's really normal to be irregular during the teen years but that is 'too' close to have periods and your periods sound very heavy. With that in mind, your best person to talk to is not us but your mom. She is there to help you. As a mom myself, I like to see teens connect with their parent as I view it as my job to help my kids and your mom will want to help you too. She's really the only way to get genuine help. :>) So, see if she can take you to the doctor to discuss this. And what is done very frequently for girls in your situation is to put them on the pill. This regulates them. Little scary for a parent to have a child on the pill but in this case, it would even out your hormones. They'll likely skip tests and just suggest that and that would be a viable option for you in my opinion. But that is between you, your doctor and your mom. Good luck!