When I was a kid, 35+ years ago we used a Sears guns that were designed that way. They are cheap guns that are supposed to pressurize the cup, which forces the material out of the gun. When they inevitably are slightly over pressurized, the bottom of the cup bulges out just like that.
I am really surprised they still make that kind of junk.
There is an awful lot to learn about different types of spray equipment, different types of paint, using it all, protecting yourself from the chemicals, and cleaning your gun properly when you finish. You have now finished lesson one. It was titled: You get what you pay for.
The next piece of junk you should buy, is a gravity feed gun from Harbor Freight. HVLP is better, but if it is a conventional gravity feed gun, that is fine, you won't know the difference anyway. And, at least have a chance of successfully completing a few projects with that gun.
Good luck.