That's kind of weird. Maybe it's their way of making what they believe is polite conversation? 5'3" and 123lbs is slender. Could they be concerned that you are heading towards anorexia? It doesn't sound like you are, but maybe they are worried. I've noticed some people seem to be a little obsessed with protein and believe that we needs tons and tons of protein to be healthy. Maybe if you started educating them about how much protein you really need and how much protein you can get from non-meat sources they'll leave you alone. If every time they bother you they get a nutrition lecture, maybe they'll get the hint and leave you alone.