betta fish trouble(?)

i know this isnt reef topic, but i couldnt find a forum on this site for this...ANYWAYS... I have a ten gallon tank with a halfmoon double tailed dragon scaled plakat betta :arg: along with a sterbai cory, 4 ghost shrimp, and 3 neon tetras. the water parameters are perfect, no aggression in the tank whatsoever... its a planted tank with a filter and a heater. well one day the betta flared at the cory for a split second and one of his gills got stuck in that position! :( he is still the most active fish in the tank and eats like a pig, with no problems breathing. it doesnt seem to bother him, but it agitates me i feel like it takes away his beauty a little on that side. still a gorgeous betta. i got him at petco as a replacement betta. i wasnt planning on buying one there but he REALLY caught my attention and i had to get him (this means he is one of the most prettiest bettas ive ever seen cuz i dont like plakats that much). dos that have anything to do with his strange problem? is it life threatening? i jsut want to know what it is cuz really weird and never happened to my other bettas...