Coparenting and relationships??

So my daughters dad and I have been split since she was 18 months. I had gone through some pretty dramatic medical issues the following months and was in the hospital the entire time. Her dad did step up and have to be the full time parent and also found a babysitter who he eventually started dating. Once I was home from the hospital I gradually had my daughter back with me as I healed from certain conditions, and now we're basically at 50/50 time now. We're pretty good at communicating and just making sure our daughter spends time with both parents. The one thing that has been getting to me though... my 4 year old is calling her dads girlfriend mommy or aunty mommy. Or saying she has two mommies lol. I think the more people in my daughter's life that love her is great but it's still odd to me she calls someone else mommy when I'm still with her 3 1/2 days a week. It's gotten to the point where the girlfriend posts on fb in vague ways like 'our little princess' and even refers to herself as 'aunty mommy'. Also, my ex's dad commented on a photo of them and said MY daughter is a spitting image of her(the girlfriend) and one of his own daughters. I don't know if I'm just being weird about it but sometimes it feels off to me and along with other small factors. I think I just mostly need to vent because I'm not sure it's anything to have a conversation with them about but this coparenting sh*t is hard sometimes..