Brown discharge, never had first period?

I'm 13 and I've never had my first period before but 2 days ago, there was some brown discharge in my undies. I put a pad in and at the end of the day there was some brown discharge in the pad. So the next day (yesterday) a put a pad in again and there was even more at the end of the day. Probably about as big as a thumb. Now today I just put a new pad in and there is already a little stain in there, but it has a reddish tint. So um is this my period? Or am I about to start it, and if so how long do you think?