HELP!? Could I be pregnant ?

I had my period 2 weeks ago..my period has NEVER been regular .. It just shows up whenever it feels like .. Sometimes I have gone 5 or 6 months without it .... Anyways ..the last 2or3 days I have had some spotting and horrible cramps ..back pain and neck pain..I'm so tired (more than usual lol) .. It is a fair amount and it's only when I wipe . 
I'm kinda freaking out a little because me and my fiancé don't use protection anymore and sometimes he has a hard time pulling out.. I took a pregnancy test tonight and it was a solid negative .  .. If anyone could help me out that would be great .. I just want to know .