15 weeks postpartum

I am 15 weeks postpartum and I breastfeed and pump while at work. It appears as though I've gotten my period, which came as a shock because it seems awfully wary for it to return. It's also really light. Enough to use a tampon but when I change it, it's not full. Does this seem normal? Today is day 4 of bleeding. Do you think its a period or just break through bleeding. Also should I expect on next month or do you think its going to be irregular, from your personal experiences. Thanks!!