Spotting in between periods.?

Excuse me for being blunt (for anyone who is easily grossed out). Today I noticed a small and really light red spot on the toilet paper so I tried it again; instead of light red it was a brownish red (like old blood after your period is done). I'm ually active (in a relationship for more than a year/it has been almost two weeks since last time I've had ) and on birth control (Yasmin for more than a year) but my period is not scheduled for another week. My period doesn't come on the exact same day but around the same time of the week, so it is pretty regular. I've never had this happen though and only once did I have two periods in one month (which the gyno said was from stress). This week was the first week of school (I transfered to a new college), and I have been tired (not on a major scale, just not getting enough) so I don't really feel stressed out but I am wondering if it could be a factor.