Should I see a doctor?

I'm 16 and I have very irregular periods. I've gone up to a whole 5 months without one before, to a couple of weeks. I started my period when I was 11, and it has been like that since.

The other thing is that for the past year or more, my period blood has been black, and there has hardly been any at all. My periods last for up to 3 days sometimes shorter, hardly ever longer. I also get no symptoms, no pains, cravings or other. So recently told my grandma (who I live with) about it, and she doesn't know either. Does this happen for anyone else?? Is it normal?