My son doesn’t want to go to my mil /his grandma

So we were on vacation for couple weeks outside of USA and my mil called us often on FaceTime to see my son/her grandson.And he was doing fine over phone he didn’t sit for couple minutes to see her he would call her grandma and blow kiss and go his own way (he is 18 months old).

So now when we came back home we visit mil and my son didn’t want to be alone with her (I went to buy some stuff and I leave him with her as usual and he was fine before).He was crying entire time until I came back.

And now she is trying to convince me that he isn’t good by his behavior how he avoids her,and how his neck is so thin it can almost break 😳.

I’m telling her he needs time to get used on her again he didn’t see her face to face for couple weeks and his neck is fine he is growing boy getting taller and can’t be chubby like baby when he was born.

But no she is still saying something is wrong with him.

Like whaaat

Does anyone have any advice how to approach this without big arguments.

He is doing great with my niece and her boyfriend and boyfriends mom and he didn’t see them also for same amount time.