Infertility; loss; hell, breakups. They're all dreadful. As anyone can tell you, sometimes the nastiest part is when you think everything is fine and life decides to remind you of what you haven't thought about in a week, a month, a year.
The little girl and her daddy out to brunch on what is obviously an auspicious occasion. The picture your friend posts of a most memorable interaction with her growing baby. Yet another pregnancy announcement. The pretty wedding photos of people who are much happier and much more in love than you ever were.
Whatever your first feeling is, that is both normal and absolutely okay. It hurts to be reminded of what is absent. But don't dwell and don't perseverate on why did she post this when she knows what you're dealing with.