Why would someone who's healthy, fit and slim, not support their loved one's weight loss journey?

I'm having this issue with my mother. For some reason she's completely opposed to me losing the weight I gained last year. She's very active, eats right and has always been fit. Why would she not want me to be fit as well?

It saddens me that I can't share this with her. We're very close and see each other often, so she's already noticed that I've lost some weight. She thinks that it's purely because I'm being more active and going to the gym. When I mentioned that I would be happy to lose a little weight, she said that I shouldn't lose weight at all because I'm fine as I am. Yes, my BMI is technically in the "normal weight" category, but that's irrelevant to my goal. I want to be fit. When I told her that, she repeated that I'm "fine already" and said that I shouldn't "get these kind of ideas into my head".

I don't know how to deal with this. I'd like to tell her that I'm actively trying to lose weight, but I'm concerned that she would react poorly to it. I don't even understand why she's against this in the first place. Frustrating...