Ive got an underfed snake too and its been gaining like crazy since ive had him. He eats 80g rats once a week and hes about 600g right now. Underfed animals in general will put on weight rapidly when put on a stable diet. Just like people. Get a person who is sick and cant eat very much, they can lose 5-10lbs in a few days to a week. But once there better and eating the right proportions again, the weight goes right back on in just a day or two. Your snake is just catching up on lost time and meals.