I don't have this problem, but I have heard of women having this problem. I don't know if it's AS related. Severe periods are not typical, some pain is normal, as are headaches, backaches and stomach upset.
It could be how your body discharges the blood. I used to have some pain (only for short times though) and my Mum said it was because the blood was thicker than usual, I don't know if that's totally true but it's worth asking about.
A hormone check is a good idea if you can get one (and think somethings wrong in that regard). Hot water bottles can sometimes cause cramps to ease a bit, and painkillers can help. I don't know very much about this (I know I should). A doctor might be able to assist. I used to eat some fresh parsley and that seemed to help me, but it could have been a psycological effect.
I hope you find a solution for this soon, it sounds very hard to deal with.