Very light spotting, need advice!

I am 36 weeks and 4 days. Today I have experienced two times where there was VERY light spotting on my underwear (like, the amount you get from knicking your leg when you shave). I'm a B- blood type so they have me nervous about bleeding anyway but I have had that Rhogam shot. There's no mucus, just light spots of red blood. Should I call the doctors after hours or is this normal??