This is not such a difficult issue. You need to do two things:

Increase the size of the prey (should be on adult mice at that weight, regardless of age), and have a fecal done. Most reptile vets will allow you to just bring in the fecal for examination & the charge is minimal ($15-$20). If your BP has parasites, then it will cost a bit more for the treatment, but as keepers we are all aware that vet expenses come along with the territory.

Hopefully, this is just a matter of the prey size being too small, but it is best to absolutely sure that parasites are not involved.

Best of luck.