I woke this morning to find my big old snail, Frank, whom I got from Andy earlier this year, had passed away

:
Im very curious, however...his body was incredibly swollen, and there was a huge 'bubble' sticking out of his shell. I dont know how else to describe it, it looked like some part of him got inflated and was sticking outo f his shell. ANyone know what this is?

I thought water params, right away, since he was swollen, but i checked, and all is fine except nitrAtes were a bit higher then what I like to see, and all my others are jsut fine munching their cukes...