severe cramps, period that lasts about 10 days, on new pill

Hey doctors! (:
I am on a progestin-only pill for birth control, though I have not been sexually active for over a year. I recently changed to a different brand because my insurance doesn't cover my other brand anymore. Anyways, my old pill made me get periods that were very light and I had minimal cramping. They also would only last about 3 days, and then stop. However, since I started my new pill, I get severe cramps and a period that lasts about 10 days. When my cramps start, they come on suddenly, and I feel almost like I have a knife in my stomach; I can't move, breathe, sit, stand, or lie down. I've actually thought that I had appendicitis a few times. They subside after a few hours, but then a few days later I get new cramps that are a dull achy feeling, but the ache is in my back, my lower quadrants of my abdomen, and radiates down to my knees. The weird part is that I don't get my period sometimes, and other times, it comes about a week after the second cramping.
Any ideas?