Why am I cycling every 14 days when I am exclusively feeding?

My son is almost 5 months old, and I am exclusively nursing him. He is getting enough food, and feeds every 1-1.5 hrs. When he sleeps for about 8 hours at night, I pump after about 4. I am dieting, excercising 5 days a week, and on the "mini-pill". so, after 5-7 days of being period free, I have regular 7 day periods, at first with very heavy bleeding and sometimes clotting that is about the size of a 1/2 dollar (or a little larger at times). this has been going on for 3.5 months after the almost 2 1/4 months of constant bleeding after having him. I am mentally a mess every other week (hormones, I think). IS this normal? I lost my insurance, and can't go to my follow-up. I had a history of irregular periods, and was supposed to get my thyroid checked, as well as an ultrasound to see if I needed an DNC. I am experincing heavy cramping, worse than before I had my child and was cycling. Please help me with ideas! thanks