What Causes Brown Discharge During Menstruation?

Hi. I am 35 years old and have two kids, 5 yr old girl and 2 yr old boy. I work out 3-4 times a week and my eating habit is relatively good. My period cycles were pretty regular and the bleeding always stopped within a week. These couple of months, periods started but the brown discharge continued for 10-15 days after the bleeding stopped. I also noticed that my weight gained 4-5 pounds and always feel so bloated all the time. I went to see my OB and did my annual exam (pap smear) and talked to her about my concern and she suggested that I should check my thyroid level. The pap and thyroid level came back normal, but I feel so tired all the time and has no energy and lately have been having this bad headache (back of my head). I don't know what is wrong with me and what I should do next.