Fortunately lots of people have been kind - and sensible! And told the OP she is not fat. (Because she isn't!) But there have been a few batshit responses. Including the classic 'we've lost sight of what a healthy weight is' type comments! Utterly bonkers.
And yeah, as you say, no wonder young girls/young woman have eating disorders. I was told I was fat (at 5 ft 3 and a half, and 9 stone 5) by a boyfriend - when I was 17, (he was 20,) and it send me into a spiral of crash dieting.
I was chubby at school (11 and a half stone,) and lost 2 stone in the 6 months after I left, and starting dating this bloke at 9 stone 5. I thought I was fine, and was happy with my weight, and my body. I dropped from size 16 to size 10/12. Yet 3-4 weeks into dating me, this bloke started poking at my waist, and telling me he could pinch an inch on me and I needed to go on a diet. As I said, it set me off on a spiral of dieting and starving myself, because I thought 'I am still too heavy, I still have fat on me.'
50-something me would tell a man who said that to go fuck himself, but I was timid and shy then, and had had no boyfriends, just a couple of dates with a couple of lads. So I took it to heart.
So yeah, this 'you're fat at 10 stone and 5ft 4' type of comment is cruel and dangerous!