What could be the reason your spine is not showing? My bf said I'm just fat?

I haven't really seen myself from behind but the other day when I was sitting only my underwear on my boyfriend said to me "wow, I can't even see an outline of you spine". I asked what he means and he said "I think you've just become so fat" :( Does it automatically mean I'm fat if my spine is not showing off? :/
What could be the reason your spine is not showing? My bf said I'm just fat?
Post Opinion