A mass on her leg is not likely related to seizures, but let the vet confirm that. I haven't study Prozac recently but some neurological drugs have more side effects after long term use, including seizures. And some neurological drugs have more side effects if the dose is lowered or discontinued.
If the mass feels like a solitary lump (as opposed to integrated into the tissue like a bad bruise) that usually has a better outcome. I've seen several dogs get fatty masses under their skin. My late Pyr, Roxy had a couple of fatty masses when I rescued her, one at her ampit. The vet could have removed them, but they weren't painful or restricted her movement, she was old, so we let them be. My mother's dog has one that we just keep an eye on to make sure it isn't getting bigger. Even to get a biopsy usually requires the dogs go under anesthesia, which I try to avoid, especially in older dogs. But as long as it's a distinctive mass, if you choose to remove it, the surgery is easier.
Good luck