bp rescue gaining and then losing weight

[ballpythonluvr]

I have an unsexed normal that is supposedly two or three years old. When I got this snake it was only 206 grams and is now 311 grams. It constantly gains weight and then consistently loses weight. I am wondering what is going on here. This snake eats like a champ on f/t rat pups. Should I increase the prey size? I feed this snake every five days. I am concerned that it could have parasites. The poop looks fairly normal and healthy. I am considering taking the next poop sample to the vet for a fecal.