It seems very soon to me? With DD I bled for 7 weeks after the birth and then didn't have a period for over a year. This time I stopped bleeding after 3 weeks but have got what appears to be my period at 6 weeks?!
DS is breastfed but for the last 2 weeks has been sleeping for between 6-9 hours at night so I'm not sure of this has any bearing on it.