MIL Comments on Eating and Weight

Thope08
I am 15w4d and my MIL insists on asking me (very frequently) how much weight I have gained. It makes me uncomfortable, because her reactions range from responding as if I am not gaining enough weight, to sheer wicked pleasure that I AM gaining weight. Last night we were eating dinner and she said, "I just enjoy the fact that you are actually eating now." 
(As a side note: I'm 120 pounds pre-pregnancy and have naturally never been a big eater.)
 
I didn't really know what to say back to her. I ending up saying something stupid like, "I have got an appetite!" But in hindsight I wish I would have said something like, "I have always eaten my fill." Or "I never starved myself in the past."
I don't know, her comment seemed almost gloat-y and made me uncomfortable and angry. I don't mind in the slightest that I am gaining weight. She just seems fixated on it. How would you have felt and what would you have said?