PREGNANCY? HELP

I'm pretty worried that I could be pregnant. 
I have taken 10 pregnancy tests over the last 18 weeks, the have all been negative. When I went to the doctors because I have had low abdominal pain, a sore lower back and frequent urination. The doctor did two pregnancy tests that also came back negative. Because they were negative and I'm still getting my period they also ordered a pelvic ultrasound to check for any cysts or problems, nothing came up. I then had a pelvic MRI which also showed nothing. 
I can now feel a round long lump when I push down on my stomach it is on my left hand side between my belly button and Pelvic bone but not in the middle it is off to the side near my hip almost and I can move it (you can't see it) 
I have period like cramps EVERYDAY for 18 weeks now. 
I last had sex 18 weeks ago and 2-3 days after that I had symptoms. I'm on the pill (i take it correctly) and my partner wears condoms everytime. We don't think we ever had any accidents!! I'm still getting my period once a month. 
I HAVE POSTED THIS SO SO SO MANY TIMES AND I MUST BE SO ANNOYING BUT I REALLY JUST DONT KNOW WHAT ELSE TO DO. IM SO STRESSED OUT BECAUSE IM GETTING NO ANSWERS. 
Surly the ultrasound would have showed something even if I was early (6-7 weeks) and MRI would have showed something (13-14) weeks. I don't really know how MRIs work but I'm guessing if it's a pelvic MRI they take pictures of your whole pelvic area?? 
Please offer me some advise! I have been to so many doctors and had so many tests. I just don't know what to do.