They are being unprofessional, to put it mildly. I agree with @Afraser with commenting politely of course on the unprofessionalism they are displaying.
It is bad enough with people we know friends/acquaintances making ridiculous comments, but we should not expect those comments coming from the medical people that are testing / treating us. Take care and just make sure you have the last say when they make these presumptions about any of your scars. It would really annoy me also to be honest. A tummy tuck is presuming you wanted cosmetic surgery to flatten your tummy, perhaps they could have just asked you what was the scar from.
On a side note, the year before I was diagnosed with breast cancer I had some surgery on my thyroid, and I have a scar on my throat area, and I had a complete stranger ask me did I cut my throat. I replied jokingly, yes I slit my throat. :)
What they are saying to you is not funny, but you could also point out their stupidity of making such an assumption. Wishing you all the best with your next appointment, and I hope you put them back in their place. xx