Swimming Calories and Losing Quickly

buniphuphu

Due to health restrictions (I need a hip replacement and a spinal fusion), my doctor has told me that the only exercise I am allowed is swimming. He doesn't even want me doing extra walking because of the damage it would do to the cartilage in my hip. I've been lackadaisical about maintaining a swimming routine because work/family take up so much time, but I've been on break this week and have gone swimming at the gym 5/9 days of break. According to MapMyFitness, I'm burning just over 1000 calories per half mile of swimming because I'm very slow, so it takes me a while to finish the workout. I have been eating back at least half of these calories per day, and weight is dropping off of me daily. I've dropped 15.2 pounds during break, which I know is too much, even though I have a lot to lose.

Could the app be under-estimating the calories burned? Should I just be grateful for the losses, knowing that they will slow down later on? Should I eat back more of the deficit?