My Betta had something similar
Recently, my betta had the same symptoms.
What was different was that on Thursday he was perfectly fine, and on Friday (the very next day) he had a lump on his head and was sluggish. It happened SO FAST I didn't know what it was!
There was a misunderstanding with some sellers and they sent me 4 nerite snails instead of two. I had 5 Nerites in there, and since they have a huge bio load, my tank started cycling again. I added Seachem Clarity that Wednesday to help the water cloudiness. It helped, but added a little bit more that Thursday. I don't know if I was supposed to do it one day after the other. There are no instructions on the bottle....This is not to say that it's Clarity's fault.
I came to the conclusion that it can't be a tumor because it appeared WAY too fast. I figured it was probably a bacterial infection. During cycling, bacteria are blooming, etc.
My Betta is in a 10 gallon tank, with 3 Nerites, 2 Assassins, and 6 Amano's. I feed him Aqueon Betta food once a day in the morning.
Anyway, enough rambling. Here's what I did.
Friday i did 30% water change and added API freshwater aquarium salt.
I also raised my water heater to 85˚. Since then, i did a 25% water change every day. I didn't add aquarium salt each time. Just on Friday and Today.
Here are pictures from Friday when the lump first appeared, and from today (Tuesday). I feel like the pictures don't do it justice. The bump was big and his 'lips' were protruded out like a duck! (haha). He is doing a lot better now! He is more energetic and has his appetite back!
the scales where he had the bump are still roughed up a bit, so I will continue to
do water changes every day, until he is back to normal! :)
I hope this helped! I tried to put the pics up in order. The first ones should be from Friday, the last two from Tuesday. :)