Right or wrong.

Fleur20 is correct - his child, his problem. We help when we can and you should not be at their beck and call.

Crikey - how dare your son act that way? I'd be very annoyed with him. No car - get a cab!

I may be completely off the mark but I wondered if your OH is DS’s father? DS is being unreasonable and may be upset that you prioritised OH over himself and his family. Having said that you were absolutely right to stick to your guns. You could have stayed in all day and not been needed. I agree with Cafe that he should take a taxi if necessary.

you are not his staff.
he needs to have contingency plans; get a taxi, or ask friends or neighbours to drive them to doctors.
anyway, with a tummy problem it's usually advisable to isolate.
perhaps you do too much for them with childcare, and they've started to take you and your services for granted.

Was there any reason why he couldn't take his two kids to the doctor?

You have the right to decide what to do with your own time.

Your son is wrong - you are the Grand parents- and surely a child wants it's parents when they're ill.
Hope he's calmed down x

He is a rude man and you have your own life to lead. You did your bit raising your own children, he has to take responsibility for his. I hope he has apologised to you because he owes you one. You sound like a lovely, caring grandma and he is so lucky to have you. Not worth falling out over but stick to your guns !

Your son is wrong. You don't have to be at his beck and call and are allowed to have an outing. You do a lot for them already. Your son can take an uber or taxi if he doesn't have his own transport. 100% on your side here.

I trust you got a proper apology.
His child....his problem.
He is very lucky you do the child care you already do.
If he throws another tantrum like this you need to reconsider the entire arrangement.

You are not wrong in taking time for yourself and saying no.

Of course you weren't wrong to say you had plans. Your son did back down later though and presumably was stressed in the morning,so I would be inclined to forget this.