Brown discharge but defo not period?!

I take the combined pill which I have for 3 weeks each month and then a 1 week break for my period.. I'm currently only on the second week in the packet but have just started with some brownish discharge which i would normally get at the start or more likely end of my period.. It's not even close to when I'm due on.
I have had unprotected sex once during this months set of pills and missed one last night but took it this morning which is actually still covers me.
Anyone got any ideas of what this might be?