Why am I spotting a week before my period?

Jaclyn
I began spotting on day 19 of my cycle. It's very light, and it's a brownish color. It doesn't have any red tint like my normal period. I'm on day 21 now and still experiencing spotting. I know it's not my period because it's not red at all. Its very light, like too light for a tampon but I still have to wear a panty liner. 
Any ideas what could be causing this? Also, I'm not sure if it's related but I started back on my birth control this cycle after being off it for 2 months. Could that cause spotting?