Spa bulges in one area what cause this and can it be fixed

Is this a Fiberglass or Acrylic Spa?
Or are you referring to a portable toy spa that is inflatable?

If the former, your shell surface is delaminating, from the rigid FiberGlass that was sprayed onto the acrylic or fiberglass shell to ridgidize it. What usually happens is "Air" is not completely removed during that spray process and over time, the acrylic will bubble due to the heat in the spa as the air expands. That Acrylic is barely paper thin when stretched out into the mold you sit in. An 8x8 sheet of acrylic is only 1/4" thick, then it is heated and placed over a tub "Mold" and then Vacuumed into what is your tubs shape that you enter into. You can imagine how thin it gets. When it goes through the ridgidizing process, All of the Air needs to be removed, sometimes it is not, and over the years it becomes evident that it wasn't.
What can you do? Well, besides that it is not very pleasing to look at, there is no problem or danger of any leaks, not yet anyway? You can break the bubbles out and find a supplier that sells acrylic repair kits in the color of your spa. You can google with the proper search terms & eventually find a kit. Maybe something like the link below shows...
Large Granite Spa Hot Tub Acrylic Repair Kit

If the latter, Sorry I don't know....

Good Luck