If you found out that your ex had a baby

Today my best friend told me that my ex BF's mom had a baby. I was very happy for her because I knew she was unable to get pregnant (she has a T-shaped uterus, I believe). After looking at the pictures and captions I noticed that it was not actually her baby.. It was her grandson. And after realizing that it was my exes baby I just was in shock and I have been feeling weird about it ever since. I don't have feelings for him we haven't talked in over a year, and haven't been together for 3 years. So I don't know why I feel like this. I can't stop thinking about it.