Betta Fish: Bulge on my betta?

I just noticed today a stange bulge on the side of my betta when i was feeding her. Im really scared, i want to know what it could be and how to treat it. I have noticed she had some odd scales around that area but i didnt pay much attention as i thought she may have just accedentally scraped herself on something and it would just heal. A few weeks ago she had finrot after i screwed up the cycle of the tank, but it was all back to normal and i managed to treat her in a hospital tank, and over time the tank also cycled back. She only has a small nerite snail as a tankmate, and i dont see any signs that ammonia might have built up again. I just changed some of the water, her tank is a 5 gallon (20 liters) and the temperature is around 25°C. Shes still active as usual, i couldnt even take proper pictures of her because she wouldnt stand still, but im worried it could be an internal tumor or organ failure. I have lost too many bettas, i cant loose her too, she was finally doing well…