Brown stuff in my panties?

Hi, 

i am 13 and I found this brown stuff in my panties, my friend said it was a light period, (plus it had a little bit of blood in it) so I got a period tracker on my iPad, and today's the day I am expected to get it again. I have not yet got it again.

Also, back in Novembe, I did get some blood on my pad and then it didn't come back untill last month (March 2015) so my question is, what was the brown stuff? And why did I get blood in Nov. and haven't got it again for a while? And last: should I be worried if I don't get it again today or tomorrow or soon?