HELP! Giant lump on Betta fish's side!

My female chocolate Betta has a big lump on her side. It formed today (I think) and is covered with scales. She is in a 5 gallon tank with a school of neon tetras. The tank is heated to 80 degrees and is filtered. There are plenty of fake plants, etc, and I test the water and do weekly 25% water changes. Just today, I noticed a large bump on my Betta fish's side. There are scales on it, so I'm thinking it MAY be a tumor. Please help! The fish is about 3-5 years old.