People always assume im anor*xic while im not and its upsetting me

I've been underweight my whole life, born with a very low weight from the start. Im 19 and have a bmi of 15, (sometimes lower for no reason) despite eating plenty and never restricting myself, but nobody believes me except my family obviously who saw me grow up and know what i eat and how much. Medical records are fine and im still trying to find the reason, but everytime im at a checkup or going out i see people giving me 'concerned' looks and nurses always laugh and say they thought im st@rving myself. I dont wanna be seen as one of *THOSE* people. Now im insecure and i cant go outside because i know people will think im one of them. I hate them, i dont want to be thrown in the same bag. Now i cant help but assume what my friends think of me... Im pretty sure nobody believes me when i say im actually healthy and eat a lot. Even my own therapist doesn't believe me. Im very depressed and frustrated because of that, it's driving me insane.. i might be exaggerating, but it still takes a toll on my mental health since i already have bad social anxiety.