My sister keeps making comments about my weight?

I recently moved in with my older sister, and she's started making comments about my weight. I know I've put on a little weight since its that time of the months and just in general, but I don't feel like its that bad. She keeps saying that I need to go to the gym with her, or when I eat she calls me a fatty. She says it jokingly, but it really bothers me. She's very extroverted and I'm the complete opposite so I don't know what to say to her. She'd probably just laugh if I said it bothered me.

It's gotten to the point where I feel kinda gross when I eat anything. I just feel fat for eating. I'm 5'4 and 119lbs. I know this isn't fat, so I don't understand why she keeps saying that I'm getting fat, or that I'm chubby. It's really, really bothering me. Like if I say my pants are tight she just laughs and says she could fit into them or something like that.

I really don't know what to do about it.
My sister keeps making comments about my weight?
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