[flamesabers]

If you are anything like me you shirt sticks out at your nipples and then hangs straight down, possibly with a valley in the centre.
I suspect that's what he picked up on, probably at an almost subconcious level?

That describes my appearance perfectly! With such an appearance, it makes sense why someone would think I'm skinny. I wonder if this is why over the last few months my mom has been telling me I'm skinny and asking me if I'm eating enough food.

I've always assumed that as my growth becomes noticeable, people (especially my family members) would be inquiring about my health and/or hormone levels, rather than how skinny I am. This is a rather unexpected turn of events for me.

I realize it may seem like I'm making a big deal out of nothing, but I feel like this is some sort of rite of passage.

I don't think you are making an unnecessarily big deal out of it and I can sympathise with the rite of passage feeling. It is somehow weirdly reassuring to know that they must be real and not a figment of your own imagination, because somebody has noticed them, even if that recognition is itself not very welcome!
I remember the first time it happened to me, I was at an Egyptian airport. I came through the scanner and turned to collect my bags, which stretched my shirt across my boobs and I saw a security guard clock my chest. He came straight over and frisked me to check if I had anything concealed under my shirt. I was soooo glad I wasn't wearing a bra, but unwelcome as it was, I felt strangely elated as well!

I would feel uncomfortable if not bothered by being frisked by a security guard.