Need advice here MIL called me fat

So im 28 M in a relationship with 28f we have been dating for quite some time now and now we are in talks for marriage ,
Yesterday was her birthday and i called her at 12 am to wish, after 15-20 mins of us talking her mother who was in another room sleeping came to her asking , how many people have called u uptil now and normal details.
After a while she goes “did the fat guy also wish u” by that she meant me, so i went on silent mode and cut the call.
Later my gf tried talking me iut of it by saying she’s just joking, and she respects me,
But i feel she doesn’t and this joke might have been circulated in her other relatives as well.
Need arvice on what to do here.