What Causes Spotting Prior To Onset Of Menstruation?

My husband and I are trying to get pregnant. I get my period pretty regularly (usually between 29 - 33 day cycles). Lately I have been noticing light pink & brownish blood/discharge on the toilet paper when wiping after peeing (not in my undies, only on toilet paper) about a week before my period is due. This month I have noticed this for the past 4-5 days (my period is due in approximately 3 days). I have also experienced little twinges in my abdomen mainly on the left side which feels like ovulation pain however I would have already ovulated by now. I have experienced some very very mild cramps (so mild, not even painful). I did a HPT this morning with my first pee and it was negative. Could I be pregnant? What is this spotting/discharge and why would it be happening?