What Causes Abnormal Menstrual Bleeding Along With Headaches And Back Pain?

Hi, I just recently had sex with my boyfriend, twice on Sunday. Monday was fine and Tuesday afternoon I started bleeding so I put on a pad. Wednesday morning came and I jumped up to the bathroom because I felt the blood coming and I just figured I was on my period which was normal for me. I peed blood and put on a pad but the blood was light. As Thursday came the blood was still light to the point where I could where the same pad all day which isn t normal for me. The blood would be red at one point but when I wiped it was pink and then sometimes it would be red. My lower back is hurting, I have headaches, haven t been eating all of my food lately until today, and I also have a brown discharge.