Really horrible behaviour by her, OP. What new mum wants to be worrying about their weight or shape?! Your focus should be on healing your body, getting enough rest and looking after your gorgeous newborn.
Of course she’s not doing it to treat you! If that were the case she wouldn’t even have mentioned your weight and only offered to buy you some new clothes as a treat if you had actively brought up your weight loss yourself and said something like “ah I’ll have to get some new clothes now my pregnancy ones are too big!”. To even bring it up as a topic is rude, pressurising and judgmental.
Think you definitely need some therapy at some point to unpick your relationship with her, but in the meantime, I’d message something like “hi mum, thanks for buying me some new clothes but I only had a baby 3 months ago so am obviously focussing on looking after her, not on losing weight! So I’m not yet back to my old size and don’t expect to be for a few months. Please can you return them for now so they’re not wasted, then maybe we can go for a shopping trip for Christmas instead? Look forward to seeing you next week!”
then if she replies with anything other than, “oh sorry, yes of course!” I would reply saying “mum, that’s really horrible of you to say, I’m a new mum and my focus is rightly on my baby, not on shallow meaningless things like my appearance. As X’s granny, I’m sure you understand that’s the only important thing right now? I’d really appreciate it if you could keep judgmental comments to yourself as it’s actually really hurtful not to be supported by your own mum at such a vulnerable stage.”
again, if she doesn’t apologise, I’d say “ok, I think it’s best if I avoid visiting until you’re in a kinder frame of mind. I just need supportive people around me at the moment, not people who are going to insult me and try to make me feel bad about myself. Let me know when you’re ready to drop this topic and then hopefully we can arrange something”. My husband would also be calling at that point and giving her what for!
best of luck, OP. Please stop fasting and enjoy this precious time with your new baby.