Being told I'm underweight because I'm vegan?

My mother suggested I see a doctor to aid me with progressing with a healthy vegan diet which I said was fine by me. My mother has never approached me before with much concern over it which I was always thankful for.
Anyway, this doctor told me that because I’m 5ft 6 and 7 stone 4 I am underweight and she has ‘major concerns.’ This is strange because I neglected my weight and health before I was vegan and have put on 6 pounds since transitioning. But they didn’t take this into account of course and told me that I was not getting any protein which I know just isn’t true.
I didn’t want to sound like the person who always argues with authority but I couldn’t help but feel offended when this doctor told me I cannot do ANY exercise due to my weight. I don’t workout that often and have always been the type to just go out for walks.
Now my mother seems convinced that I’m trying to lose weight and this doctor seemed convinced I was unhealthy and not eating enough when I told her I eat rice stir-frys and vegan pasta-bean curry and that I don’t skimp on lentils and healthy sugars. None of this made a difference though and she suggested I go back to pescatarian.
(I also always cook my own meals and have used my own money to stock up on tinned pulses and beans)