Am I wrong?

I'm frustrated. Quick story. I am a single parent to 2 children. My mother often says she will help by keeping them from time time but when she does as soon I as leave she always needs me to hurry back over nothing or will often times change plans or make plans with someone else and not say anything.

It's been awhile since I've been out and I don't ask her to do anything usually but she offers. I asked her to watch my girls while I hang out with someone I'm interested in. She says yes. An hour later after feeding them and them being excited to see her she texts and says oh you'll have to get them tonight because I work super early. I feel like a mad woman right now but why not say that when I asked because she offered to keep them the entire night and then this!!

Am I wrong for being upset?

I know my kids are my responsibility and I'm not upset about that. I'm upset that she doesn't mean what she says when she offers it!