Hey Bobby, long time so see! I'm really sorry to hear about Granny, she's one of my favorite frogs on here!

I agree with Heather, I also don't think it's an infection. A cyst is possible too, but as Grif said, they're usually on the skin. Best case scenario is its a large, fluid filled cavity and can be drained. Worst case is a tumor, and hopefully it will be able to be removed safely. Either way, a vet visit is in order...

Best of luck! Please keep us posted...