Hiya,
Unfortunately, the only things I can think of are encysted nematodes, furunculosis, TB, or tumors. If it is tumors or TB the fish will probably die.
TB will manifest in additional swelling, deformation of the spine, and wasting of the fishes tissues, if this occurs it is best to destroy the fish as quickly as you can and do several thorough water changes, use gloves when handling the fish anf wash with antibacterial soap.
Tumors can be benign or not, I know of no cure but if benign the fish can live a fairly normal life, if malignant the tumor will grow or more will appear and eventually it will die, however they are not contagious.
Furunculosis is ussually a bacterial disease (gram-neg) and can be treated with either sulfa drugs or most wide spectrum antibiotics, the bump will develope a pimple like head that will rupture, if this happens swab the wound with iodine solution and treat the tank with a topical antibiotic like Binox.
An encysted nematode is hard to actually treat but is not usually a serious problem, the worm causes a bump that usually gets a bit smaller and fairly hard to the touch then stabilizes. In most cases it kust stays that way, this type in infection is rarely contagious and can sometimes beeated with antiparasitics like a combo of metronidazole and levamisole.
Guppy
