If its soft and feels full of fluid it could be one of two things. 1) A bacterial infection which will need a vets attention to prescribe antibiotics. 2) It could be a benign cyst which may burst and still need a Vets attention. If it bursts it will place the frog at risk of infection.

EDIT; Answer these questions and repost the answers here http://www.frogforum.net/pacman-frog...enclosure.html