Gaining

I'd say just sit down with her and tell her how you feel. Maybe wear something so she can't see the weight you've put on as that might make her angry.

If she hates it as much as you say, just see if you can get her to understand your view. Yes, you've put on weight and yes you know about it, she doesn't need to be unpleasant towards you.

She would probably be in a similar situation, not wanting to have a bad relationship with you, but thinking she is trying to help.

Ive been kinda in the same boat as you, except I decided to gain more intentionally, as in the 100 pounds variant intentionally, and my mom is always on my ass about dropping the pounds, the way I usually get around it is just going through with her scheme, ( excercising, w/e ) and just continue on your way. Personally I find that Idk works perfectly for a lot of scenarios, "how did you gain so much" idk, "why do you have heavy cream in your fridge" idk, " why are all the buttons on your shorts missing" idk

Regardless though it's something you have to deal with for a while, and if you ever decide to tell her she might still be salty, deep down though she's more than likely just worried about you. Because that's what my mom is worried about, she doesn't care about the weight, just the unintended side affects of how rapidly I gained.