What Causes Brownish Or Red Spotting?

hi, I have had spotting it brownish some days red others. Its on and off all day. This has been going on for 31 days. Last month i had my period(?) lasted 8 days light, not normal for me, and it was more brownish then red. then 2 weeks later this started and hasent stopped. I am on a pop and my gyn gave me low dose estra. for 14 days that didnt help. any thoughts on what could be going on. I take my pill within 30 min of the same time every day. back in Feb I was on day late pharm didn't have it. I have taken 2 pg tests both negative.