weird lump, need help!

Housing

What size is your tank? 10 Gallon
What temperature is your tank? 86 Fahrenheit
Does your tank have a filter? Yes
Does your tank have an air stone or other type of aeration? No
Is your tank heated? Yes
What tank mates does your betta fish live with? None

Food

What type of food do you feed your betta fish? TopFin color enhancing pellets.
How often do you feed your betta fish? Once a day, 4 pellets. Fasts on Sundays.

Maintenance

How often do you perform a water change? Once a week

What percentage of the water do you change when you perform a water change? 40% New water 60% old water

What type of additives do you add to the water when you perform a water change? 7 drops per new gallon of Bettasafe

Water Parameters:
Have you tested your water? If so, what are the following parameters?

Ammonia: 0.02 ppm
Nitrite:
Nitrate:
pH: 7.2

Symptoms and Treatment

How has your betta fish's appearance changed? Tiberius has developed a lump on his left side, approximately were his intestines/stomach is. His color is lightening, becoming a white-gray in places. He also has stress stripes.

How has your betta fish's behavior changed? A bit lethargic, but otherwise the same.

When did you start noticing the symptoms? A week ago
Have you started treating your fish? If so, how? At first I assumed he was constipated. He's had issues processing his food in the past. I fasted him for two days, but nothing changed. I tried a few peas, he ate them, and pooped them out no problem but still the lump won't go away. I also added a few drops of APR Melafix for bacterial infections, just in case. This did nothing as well. I've upped his water changes to twice a week, 30% New water 70% old water.
Does your fish have any history of being ill? A bit of past constipation.
How old is your fish (approximately)? I bought Tiberius from a Wal-Mart two and a half years ago, I assume he was about a year old then. He already had his color and was fairly grown.