What Causes Bleeding After The Periods?

I am 27 years old, married with a 4 year old. My periods are fairly regular at about every 21-23 days. My husband and I are safe (condoms) biut I'm not on birth control. My period started as normal, lasted the normal 5 days and now has continued to 14 days. The first 5 days were normal bleeding and it has gradually reduced to me not needing a pad or tampon but having a brown discharge whenever I wipe. That's been the last 4 days-today after I finished exercising I now hoave bright red bleeding again although not enough to need a tampopn. I have very minal feelings of cramping and no other pain. I'm overweight for my height but at about a 28 BMI.