Brother in law and BF comment hurt my feelings

My fiancé and I suffered a miscarriage in February. Today his brother was talking about how his son is misbehaving and he says to me “one day you’ll understand” then my fiancé says to me “one day you will understand” he said this because he has a child already from a previous marriage.

I burst into tears because I lost my baby. He came to me and said I that he was so sorry and that he didn’t mean his comment in a malicious way.

I took his comment like haha I have a kid and I understand but you don’t. I don’t know if I’m overreacting but I wanted to post on here because he left to get me food and I wanted to regroup before he gets here.