I painted my car (BC/CC) about two years ago, and, only a couple months after the paint job, this spot appeared. It has remained inchanged since. It's about 3/4" x 1". It's like a blister, but feels solid underneath. The paint is unbroken, as if it was painted over as is. It's in an area where body panels were welded from the factory and leaded. I thought I removed all the lead, but maybe I didn't.
The problem is how to fix it. If I resprayed the whole "panel", that would include having to go all the way around both side panels of the car. Since it is between a seam and an edge that's about 2" wide, I was hoping I could feather it out without having to reclear 1/3rd of the car.
To be honest, I can't really tell much by the picture. I think what you are going to have to do is to determine weather it is a bubble caused by oxidized (rust) metal below bondo, or an adhesion problem. My sense is that if it were an adhesion issue with the paint, it would have some 'give' when you pressed it with your fingers. Because you say it doesn't have any 'give' to it, that leads me to believe that the bubbling lies below the paint: filler, lead, etc. It's not uncommon for filler to bubble like you described. If any water or moisture gets in behind there, it can happen in about a month or two.
You're probably going to have to flake it off with a sharp screwdriver and work into it and do whatever paint repair is required. Yeah, it sucks, but, we've all been there. Let us know what you find.
Whatever it is you will need to remove the paint to correct the problem and as soon as you remove the paint you'll be able to tell what caused it and how to fix it OR post a picture after the paint is removed and we'll tell you what you need to know.