So, to recap, the divorce was amicable right up to the point when your mum found out about the affair and then wanted you to be on her side, based on the fact that she felt she put you first despite her own feelings about her marriage which she tolerated for the greater good.
The fact that she said she only tolerated you for the sake of her grandson is a bit childish, and tit for tat. She tolerated a loveless marriage for you, so she feels she deserves payback, or rather an uncomplicated relationship.
She is lashing out at you because others have cut her off and her actions still matter to you, but a friend of mine quite rightly observed that a giver (you) needs to define the boundaries because a taker (your mum) never will.
Much depends how much her actions affect you. So, after a visit, are you OK, or does your mental health suffer?
You need to think about how much you can take, and then define the boundaries which could range from once a week to Christmas and birthdays, but be clear about this. Tell your mum with honesty and without animosity that you are happy to see her on these dates, but as she is being so hostile to you, it would be best for all concerned to keep contact to these times.
What a shame her bitterness has taken over her life, but please don't let it take over yours and your son's.