I'm 5'6". Earlier this year I trained for a half marathon and got down to 8st 3lbs. Almost without exception people told me I was too thin and, yes, in photos I now recognise that I looked gaunt and ill.
I'm fairly chunkily built, boobs, tiny waist and bum. Some people apparently thought I was lying about the training and was really in treatment for cancer.
Too thin is actually a thing.