My Periods Are Pretty Irregular. They Are Usually Around 30

My periods are pretty irregular. They are usually around 30 days apart but they last a week or longer. The past few months my periods have had a lot of large clumps and are very stringy and dark at times. I've also had cramps which is pretty normal for me. I was wondering if I should be concerned about a possible health problem? I have had sex in between my periods but Always with protection so I don't think pregnancy or miscarriage is an explanation. I'm not on any birth control. Please let me know.