weird bleeing

I have been on estro-step for goin on 4 months. I know that in the first few months there is the possibility of spotting mid cycle of your period. Well, I had no side effects what so ever. but now all of a sudden i am spotting what i call old blood. That brown blood that comes either at the beginning or at the end of a period sometimes. Has anyone else experincied this so many months in???