Decreased appetite after having kids

I need help. I used to eat all the time and never gain weight but I was at a healthy weight. I’ve had two children. I’m now at about 90lbs. With my last child he had a heart defect and had surgery the day after I had him. Then was on oxygen for a year. My appetite decrease to practically nothing although I’m always thirsty. I’ve talked to my doctor many times about this. He keeps telling me I’m worried about my son. He is healthy and doing great playing sports and all. How can I get my appetite back and gain weight again with out the whey shakes I hate that stuff. Thank you