I think you are underreacting to the A&E visit and lack of communication at the time.
Does your mum have authorisation to agree to any procedures a doctor might want to perform?
She overstepped her authority there.
It is actually very controlling of her to make decisions and keep you in the dark. You are not a child any more but she wants you to accept that 'mother knows best' - even about your own child and even in a health emergency.
I don't think she is really acknowledging that you are the mother of your DS here, and your DP is the father, both of you with full parental responsibility, and she is not in that category.
I think you have the glimmering of an idea that there are blurred boundaries here (you do feel anger) but you maybe have a habit of accepting her decisions and her controlling nature is familiar, so you have decided you see the logic in what she chose to do. Has she ever come across to you as a bit domineering?
Ultimately you need to start recognising her attitude as a problem and you need to try to assert yourself a bit more, or face the possibility of more encroachments on your role by her, and more dismissal of your maternal feelings, and more sidelining as happened last night.