Betta Fish: Please help! Came back from a few days gone and my betta has a sore on his body and swelling under his hill. What is this and how can I help him?

I’m not sure what suddenly has happened to my betta, under his gill is swollen with a lump and he also has this sore on his body. He’s been being more lethargic too

I do weekly water changes and testing, no ammonia no nitrate. Water heated to 80 degrees. He is in a five gallon tank.