Betta With A Facial Growth

hi there,
im wondering if anyone can help me out here. I have a lovely betta in a community tank, hes been through the mill somewhat over the time ive had him, seems prone to finrot etc. the current problem is that he now has a lump on the side of his face, which wasnt there yesterday. im hoping someone out there might have some idea about what it could be or how i can treat it.
each time he has had treatment in the past, ive had to isolate him in a little hospital tank, as he shares the tank with several snails and a whole lot of shrimps.
i am wondering if it could be a result in some way of last nights mishap.... i stayed out the night, and unbeknown to me, my filter had the power knocked off. the filter was off all night, so on my return this morning i found the water very cloudy and my 2 loaches dead (though everyone else seemed ok, im guessing it was an oxygen issue) todays been pretty upsetting all in all.
ive siphoned, and hoovered the gravel to rid the tank of the visible build up of unfiltered mess, done a 40% emergency water change using aquasafe and easybalance. tested the water to see if what ive got in there now is ok, and the ammonia is up to 2, ive never had an ammonia issue before. could this sad turn of events be the reason my betta has this lump i guess is what im wondering.
anyway...... heres hoping someone has an idea or advice.
please see the pic for a rough idea of whats up with his face:
thanks in advance

Not much of a clue but daily water changes can help in case it's some bacterial infection! See if that helps somewhat! My bettas get the odd scrape n bump from a hollow tree stump I thinks not all that smooth inside! They'd start getting worse unless I'd do the daily water changes then 3 or 4 days of doing so and all would be clear! Could be that he had a minor injury and a build up of gunk n bacteria could have infected him! don't know if fish are the same lol but having had the odd infected injury the second sign would be the swelling. Just see if it makes any difference any how it's worth ruling out!!