What Causes Chronic Abdominal Cramps During Pregnancy?

I am suffering from severe abdominal cramps. The pain travels down both thighs and across my back. The pain lasts for 20minutes at a time and it is conplete agony. I am 6 weeks pregnant and have been back and forth to hospital for tests but to make sure baby is developing ok but nothing is getting done about this pain. I have had it for the last 3 weeks and it only seems the pain is getting much worse these last couple days, and I didn t think it could possibly get any worse!. I m worried it could be some type of stomach ulcer or stones or something like pelvic inflammatory disease. I feel like I need to ring an ambulance when the pain comes one however I feel like they just fob me off and have done for 3 weeks now. They say it could be pains to do with pregnancy but I hand on heart think it s something much more serious. Do you have any experience in seeing these symptoms together? I am physically and mentally drained from it all.