inflatable

I've been tasked with creating a 3D model of a demo inflatable
structure here in our lab. The goal is to inflate it in ABAQUS, and I
have been making some progress in using the *FLUID CAVITY keyword.
However I'm noticing odd things in my results. I have the pressure of
the gas increase to 4psi in a tabular form, however as I watch the
frame by frame, the stress concentrations on the structure vary oddly,
and the model deforms oddly. Almost as if it was a ball being pressed
back and forth against the floor, expand contract, that kind of deal.