What Causes Vaginal Spotting Post Menstruation?

Hi, I had a baby in November. I had a c-section and tube removed. This is my second section. She is now almost 5 months old and if I have a period I spot for a week and Half after my period ends mainly dark brown. I did get MRSA with the last c sectio. If I exercise I spot. If I do Keagle s I spot. What is the problem ? Should I be worried?