How bad can the body dysmorphia be?

I'm losing my mind rn. I don't really weigh myself, but I've had an appointment with a nutritionist and I've been told I'm bmi >13.4< and I TRULY (and when I say TRULY I mean it) don't feel like that. I guess I picture something different when I imagine how a person with that bmi looks... Now I know that looks don't define that and stuff but I literally see myself like average looking, and I don't even feel that bad (my heart rate is >53<). And I'm awaiting for the results of the blood test I've had... But yeah, I'm still spechless and confused, I don't know how to feel. Any insight would be very appreciated. Thank you