What does it mean when a person tells you to "stop flexing" when you aren't even flexing?

[Anonymous]

Whenever I have my shirt off in public I get two reactions. Either it's "wow you're skinny" or "stop flexing". I weigh about 123 pounds and go to the gym about four days a week. Because of that, I have built a little bit of muscle, my arms and shoulders pop out more, my chest is small, but it isn't flat and bony, but my rib bones show and my spine pops out when I bend over. Usually it is dudes that tell me I'm skinny and girls tell me to stop flexing. So what does it mean when someone tells me to stop flexing?

[Junlian]

Stop showing off. If you were. There's no need to. If your weren't then tell them stop being delusional.

[Asker]

I wasn't flexing at all

[Anonymous]

Dont take your shirt off in public then. You shouldn't be doing that anyways.

[Asker]

Public places like swimming pools and stuff, not just in the middle of a mall or something

[Anonymous]

It means you're flexing, but lying about it.

[Asker]

but i'm not flexing at all