AIO or my mother an asshole for want me to take weight loss drug?

I'm not an english speaker, just learning.

Last evening my mother's friend came to us for a dinner. Later they talked at the kitchen without me, and they invited me because they wanted to have a conversation. My mother told me that from next week I'm going to take a medicine. No negotiation, it was an "order". I asked what medicine is it and why do I need it. My mother looked to her friend who is a nurse, and she started speaking. She told me that it will make me lose weight. I naively asked that how is it possible that I eat and not gain weight? She said I won't eat. I didn't understand and she said: you won't be able to. She said something like no matter how bad I want to be eat, it will be impossible for me to eat more than I should be.

I freaked out a little bit and I told them that they don't even know how much I weigh. She said that it doesn't matter, based on what she sees I'm more then big enough.

Are they an asshole or am I overreacting? I'm 1 month away of being 20 years old (female).