Nursing 8 month old, spotty bleeding 3 weeks ago, could I be pregnant?

My son will be 8 months on Friday. I experienced some light bleeding about 3 weeks ago which started out reddish the first day, but went quickly to brown and was very light. It lasted maybe 3 days total. Also the past two weeks my milk supply has really dipped and my son has been very fussy while nursing. I've been tired (but my son had a series of bad nights sleeping which could explain that), urinating a lot, and having headaches. I'm not sure if I just got my period back and it was just light and short because it was the first or whether that might have been implantation bleeding. My husband and I have been relying on nursing and pulling out. I'm concerned that if I am pregnant, because I'm RH negative and my husband is positive, that the baby could be at risk. Anyone who might have an idea as to whether I may be pregnant and if so whether I need to get a shot for the RH difference I'd really appreciate hearing from you! Thanks!