My periods are coming 3 months late. And when I do have them I get really severe cramps?

Not ovulating. The reasons for irregular periods are many, and the evaluation is usually based on your age. That is, there are different reasons and treatments depending on whether you are 22 versus 48. It is not unusual to have heavy, crampy periods after missing 1-2 months because of a build up of tissue. Most all the causes can be treated once identified.