MIL problems

So.. a bit of a rant.. 
My son was born 8 weeks early and only 3 pounds. He's 1 in 2 weeks and is now 17 pounds 11 ounces. 
He's still breastfed and eats solids, but has really been off them for the last 2 weeks. 
We are visiting my MIL who keeps asking me if my milk has dried up and saying things like your son is so weak and small  but you're so big. She'll then claim she's joking. It's really upsetting me. 
I know my son is small but we've come so far and he's done amazingly well. He had a bad chest infection for 2 weeks and lost around 12 ounces so I've already been stressed about that.