What is causing the brown spotting between periods?

I am on the pill Yasmin, and have carried on with the packets (not had a week break at the end of each one.) A couple of days ago i got this brown blood appear, sometimes its red, and sometimes i will have none, but then lots a few hours later. I did have stomach cramps, but they have dissappeared now. Anyone have any idea what this could be? I have been on the pill for a while so my body is used to it now.