Agreed. I've been thin my entire life. And have had similar experiences. "Skinny Bitch" was something I was called so often it almost actually became my name. I see all the memes about how size 0 (US size) are "real women" blah, blah, blah. all in the name of body positivity. When I wrote about it I was told flat out by a commenter that "body positivity isn't for me." That says it all doesn't it?
I too was run to doctors for years on end in the frantic search for reasons I was they way I was. The reason? I happened to be thin. I'm almost 52 now and have added a little weight over the years, enough to have me at between 98-100 lbs (depending on the season) at 5'5". Accoridng to the "BMI charts" I'm supposedly still "severely underweight." Whatever, I'm over it. It's just how I was built and if other people want to devalue my struggles etc. well, it says more about them than it does about me.