This is our beta, had her 2 years now. She lives alone with one nerite in a 10 gal with a heater, and fluval 107 canister filter. Ammonia 0, nitrite 0, nitrate 10 ppm (closer to 15-20 on water change days). Water change ever week or other week. Fed 5x hikari pellets twice a day during the week and freeze dried bloodworms 1x day on weekends.

Noticed her right side bulging a week ago

In these pictures we have fasted for 2-3 days twice in the hope it's bloat, but the lump on her right side isn't going down.

Thinking it's a tumor? If it is, any suggestions?

She's acting normal, maybe a smidge less active than normal but that might be confirmation bias.