Are there any other symptoms? Lethargy, difficulty walking, etc? Can you post a picture of this lump?
How old is your bird? What breed?
Is she laying eggs still with this lump or no? What makes you say she is not egg bound? If she is still laying through this, are her eggs normal?
Is she eating and drinking well?
Have you isolated her from the flock to monitor symptoms/intake/output?
How's her poops? Normal, runny, funny colored?
Any discharge from nose, mouth or rear end?
Any panting or signs of respiratory distress?
Antibiotics are often the first thing suggested when the person doesn't have a clue what is the REAL problem. They will not help cure a tumor, help with egg production issues, treat cocci or worms, or treat ANYTHING else OTHER than a bacterial infection. AND they can actually make matters worse when not used properly.
Personally I wouldn't give antibiotics unless I had a pretty good diagnosis. There are potential side effects and they can actually make a weak hen weaker if she is not suffering from a bacterial infection. That's just my 5 C worth.
Maybe with some more information on exactly what is happening with your girl, the folks here can help poinpint an accurate diagnosis and suggest remedies/medicines/treatments based on a more accurate guess as to what is going on. In my experience, a lot of folks here know more about common and not-so-common ailments of chickens than most vets unless they specialize in poultry.