First period?

I am 5 weeks postpartum. A few days ago my pp bleeding was brown and was to the point where I didn’t need to wear a liner anymore. It had almost entirely stopped. Yesterday I started bleeding more and it was bright red. I have mild cramping and some back aches but nothing even remotely painful to the point where its concerning. today the bleeding was much heavier but as the day is ending it seems to be lessening. I don’t have any other symptoms that can mean something more serious but I am mostly bottle feeding expressed breast milk and breastfeeding a few times throughout the day. I’ve read that a period will not return as quickly if you’re breastfeeding. What’s the probability that this is a period and not something more serious?