First period after giving birth


I had a natural birth and still solely breastfeeding my son, just over 4 months now.. recently I’ve started having these cramps with a bit of heavy feeling, accompanied by a feeling (but no symptoms) of an upset tummy. I’m thinking if it’s my period coming back? Or can I be pregnant again? I thought neither is possible while I breastfeed exclusively.. Or what else can it be?

Any experiences, ladies? Anything to share?

Thank you in advance!