What Causes Red Brownish Blood Spotting Between Periods?

Hi! I have been spotting red brownish blood for three days now. It's so light that I don't need a pad. I've never had this problem before eventhough my period is between 35 - 40days. My last period was on Dec,4th. Is it a pre-period blood? (I always have PMS signs: tender breasts, aching tummy.) But I don't have any of those at all. Also,I checked PT, it's negative. What's wrong with me?