I don't think it is a case where you have to prove your case to your mother in law (although it may be worth explaining things to your husband) I think she knows exactly how it is like, and is just being catty and manipulative.
I have had experience of these kind of people in my life for several years now, and I continue to receive undermining treatment from them, in every shape or form - If I am tired, I 'look great', if I have just come home from hospital, tired out and in pain, after a cesarean, 'you look so well, when I had my cesarean I looked crap' and so forth - and that's the least of it.
Its just little, petty, irritating stuff that can do your head in if you try to see any logic behind it..
These kind of people have their own insecurities, and seek attention and sympathy through undermining others in this way - they may have controlling/dominant personalities, and may also be jealous of the relationship between you and your spouse (or simply just jealous of you) and well, I may be reading too much into your post, but this is what I have experienced!
I just avoid making a fuss or talking/listening to them for any great length of time, to avoid conflict and avoid hearing any rubbish! I would love to be able to give cutting but humorous answers in return but can never think of anything to say at the time!