If it is a tumor, there are some things you can try. I have a female angel that had a tumor;
She was an awesome breeder, huge spawns, but stopped spawning. Angels will sometimes take a break like that, I wasn't too concerned. When she developed that swelling I got concerned, contacting other breeders & distributors of aquatic pharmaceuticals. The thing that worked was a product called clout, with daily water changes. You may want to start with that, along with adding some epsom salt to the water.
The step after that would probably be needle aspiration, which consists of draining the tumor, if possible, with a hypodermic needle. From what I have heard this does work if done correctly, but the tumors tend to reoccur, similar to having tumors removed from a dog.