2 Weird Periods - Brown Discharge

I've already made an OB appointment, but I guess I'm just looking for some reassurance because I'm scared. My baby is less than a year old, I've had a few normal periods, and then all of a sudden I miss one. Negative pregnancy tests. I miss another month, light brown discharge for a few days only. Now I am due for my period and am getting the same thing; just lower back pain, breast pain, and light brown discharge. I don't know if my hormones are still out of whack, something bad is happening with my uterus, or what... but I'm scared. Has this happened to anyone else?