Awww please don't feel like a bad betta owner, this happens to betta owners all the time, for no rhyme or reason. His tail was soooooo long and flowy, it may have just been weighing him down a little too much, and now that it's gone he has more energy to flair and be feisty!
My Current bettas:
Sullivan- Orange rose tail halfmoon male
Daisy- Mustard Gas (black and yellow) veil tail female
Milton- Teal butterfly halfmoon male (mom's betta)
DFTBA to nerdfighters working to decrease world suck!