What Causes Light Pink To Brown Spotting During Periods

Hi, I ve experienced on and off light pink to brown spotting while still on the pill two days before I should have started the sugar pills last week I was suppose to get my period but I didn t, unless the spotting was my period? I ve been having other symptoms as well, more headaches than usual, heart burn and hiccups when I wake up in the middle of the night, backache, side cramps, mild cramping, constipation, could I be pregnant? I took a test last Monday, it was negative.