Pregnant after birth control??

I went off birth control right after my last cycle.  I was suppose to start my period yesterday and I haven't. I took a pregnancy test yesterday and it was negative. I have had some lower back cramping and a little bit of cramping below my belly button but nothing serious. I had a couple spots of blood on Friday when I wiped but that's it. I didnt test to see if and when i ovulated. I'm wondering if my period isn't going back to normal after birth control. Before birth control I had a very regular cycle. Has anyone else had this problem after birth control or does this sound like I'm possibly pregnant. My nipples are a little sensitive other then that I have no other side effects.