She shouldn't derail your desire to be healthier. Keep working out and eating better. I feel like her goal was to help, but if she lost weight and is doing things that will prevent you from trying to lose weight, that is odd, and it was dishonest of her to try to trick you, so while I want to think she did this for a good reason, it bugs me.

Do your own clothing shopping, throw out any tops you don't want (or return them), tell her you don't want her shopping for you anymore (but thank her for doing it in the past), and keep improving yourself. There is NOTHING wrong with eating less and exercising more, it's actually a great thing to do and I hope she's not trying to discourage you from self improvement for her own reasons.

Who cooks the food in your house? How are you dieting?