What does it mean if a beta fish is swimming on its side and its belly is all puffed up and bloated?

It sounds like it has swim bladder disease.

here are some symptoms...

The betta is unable to swim properly, characterized by floating at the top, sinking to the bottom and/or listing sideways. The betta often looks like it takes it a great deal of work to swim to the bottom (if floating) or reach the surface (if sinking), and will pop back up like a cork or sink like a stone.

A constipated fish may look bloated (slightly or greatly), or seem to have difficulty passing feces