Your MIL wouldnt have made any plans yesterday because you were supposed to be there so now she suddenly had nothing to do because you decided that you didn't want to go. You also expected her to be free today because you decided it would suit you better. You've messed up her weekend for no other reason than it suited you. She has every right to be a little annoyed and, if you don't turn up today either she has every right to be more than a little annoyed! Go and see her, take some flowers as an apology and get over yourself. There's more to this life than you.
Her son was with her, she hadn’t made plans because he was going to be there.
OP asked and she said fine, I’m not seeing any expectations of anything. MIL could easily have said she was busy Sunday.
She “messed up” the weekend because she was unable to go. Whether or not you think she could have gone anyway is irrelevant.
OP owes nobody an apology beyond what she has done already. DH owes an apology to his mum because the correct course of action was for him to say “Ok, I’ll come collect DC and bring him over.”
I’m always suspicious when two women have a conversation and sort things out among themselves only to have a bloke later say their mum was really upset and said blah blah blah. There are two possibilities, either he is shit stirring, or she is incapable of voicing her thoughts and feelings. None of those are OP’s fault.
@wonderw when you say she doesn’t like you, has she said/done a lot that makes you feel like that, or is it only from “my mum said…”