What Causes Chronic Lower Back Pain With Irregular Menstruation?

Hi I am worried , I have had severe back paid for over 10 months and had various Chiropractors work on pain without success, I last had intercourse early Oct 2014 and worried I could be pregnant , I have taken 3 tests all negative , my GP examined my tummy a few months ago said did not feel like pregnancy my swollen tummy . I was given my 1st Injection of Depro in Oct and my 2nd in January as my periods prior had been erractic and heavy , so I have no idea when my last period was , my tummy is swollen, and I have constant period pain also , there is also talk of Fybroids