Really Dark Brown Vaginal Spotting

Hi I will try to be very specific about my situation and hope that someone can help me, my last period was on April 28th 2010 and since i have not seen another period which means it's late i have had dark brown spotting in between, I had my tubes tied October of 2008 after i had my baby and my periods have always been irregular but i have always had 1 each month just different dates which is now concerning me because i didn't get 1 for the month of May and today i noticed i was spotting but it was really really dark brown i've never seen it so dark before, another thing is i was told on my last pap that i might have a polyp but i never went back to confirm it with a special test that i had to get done because i was scared it might hurt it's some test where they have to fill up my uterus with saline water something like that idk, and now i am very worried about what could be happening i wonder if it's because of the polyp or could it be something else.........