bp rescue gaining and then losing weight

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.

Get a fecal done just to be on the safe side. I have a 400 gram un-sexed ball that takes a small rat every 7 days so I think you can move up
I believe if you want to go by weight for the food item take the snakes weight and multiply it by .15 and that should be the weight of the prey item (roughly)
So your 311 gram snake should eat a prey item that is about 47 grams.

I would stick to 10% of the snakes weight at this size, especially for a older small snake that has obviously always had eating issues.

I have 4 month old snakes over 300 grams telling me this snake is a very picky eater or has something going on *as the op suggest*