If I were caring for a baby for someone for a couple of hours I wouldn't EVER buy formula to take with me just in case. 
That woman planned this. She bought formula, in advance for a .
Even for a FF baby one would call to check to see what brand to buy FFS!
She knew what she was doing, and had the formula IN HER BAG as she crossed the threshold of OP's house.
She still didn't mention it.
She chose further NOT to call her son, or the OP to check if the milk was alright and if it was safe to use.
No. She took the FF that she had bought, in advance, for an EBF baby, in full knowledge of that fact, and threw away her mother's milk and made up a bottle.
If she went to this trouble to get at the OP, what else would she do in terms of neglect/lack of care or even wilful HARM to use the vulnerable and helpless child, her own grandchild as collateral damage to hurt her DIL, the OP.
This has to be one of the worst, most insidious cases of abuse of power i've seen since the MIL gave the baby triplets CP.
The MIL here has made her GC sick, and has to be held responsible for this. perhaps she even poisoned the FF she made up.
Given the planning she's already put in, why would she stop at mere contempt of her son's family?
I agree, having calmed down now, that confrontation won't do anything much apart from raise the OP's blood pressure, but coldly and with a measured demeanour, i'd be informing her of my new childcare arrangements the second she commented on lack of contact.
I'd expect her son to back me up too, no excuses.
This was a terrible abuse of power here. :(