Am I crazy for being annoyed?

At Easter my husband and I were talking about the baby shower. His mom asked when we were thinking of having it. I told her July 31st because I would have some of my out of town family already in. She asked what day of the week it was, I said Sunday. My MIL sits on an event board in our town and one of the events she is a part of starts the next day. I asked if having the shower the day before it starts was going to be a problem. She said no, if so, she would get someone to cover her for the pre event check ins.

So fast forward to yesterday when my husband gave her the invite. The date we agreed on and she said was ok suddenly is a huge problem. However, all the invites are done and plans are set...we've already gotten others to other people.

Am I crazy for being really annoyed that we had a conversation months ago in which she said it was ok (her duties never change from year to year for the event so she knew what she would be needed for) and now she is saying that it is a problem to my husband like I purposely chose that date to make it difficult for her?

Just one more thing that I am adding to my list of things she does that drive me crazy....