What Causes Bleeding In The Mid Of Menstruation Cycle Of A Breast Feeding Mother?

Okay, I gave birth to my daughter 4 months ago. At the beginning of the month, my husband and i had sex. I took the morning after pill due to not using a condom. I got a period two days after at least i think so. I havent had one yet due to breastfeeding. It's been about a week or two and I started bleeding again. What is happening?