Is it unsafe to inflate your belly with a pump?

If you're a physician, it's not particularly unsafe, though you should probably get another physician to do it rather than trying to do it yourself.

If you're just some schmo who thinks for some bizarre reason it would be a cool thing to do, good Lord man what are you thinking? Yes it's dangerous! You could nick an artery and bleed to death, nick your intestines and get peritonitis, or any number of other bad things!