What kind of fish? What other fish do you have with it and in what size tank? What are your water parameters (ammonia, nitrite, nitrate, pH)? Has your tank cycled? How long have you had all the fish in this tank? Are there any plants in the tank?
The first symptom is that of constipation or dropsy. Constipation can be helped by not feeding for a couple of days and then giving a shelled mashed pea to the fish to eat.
Dropsy is not as simple to cure and a lot more deadly. It is an internal bacterial infection and you will need to treat with some appropriate mediaction.
I am not sure which it is but I am afraid it will probably be dropsy. Do you have a seperate isolation/hospital tank to treat sick fish in?
The bump on it's side is what leads me to think it's some kind of internal infection. However, it could also be a tumour. Tumours cannot be treated but are rare and usualy don't bother the fish much and it can lead a normal life.
Its lethargic behaviour however, suggests there is deffinately something wrong. I would isolate the fish and add some medication to the water. Let it go without food for a day and then feed a pea. See if there is any improvement.