do you have two bettas in the same tank???
if so you must seperate them asap, one of them (perferably the male) has to go into another tank. you probly know that 2 males together is a no-no, but even though ones a female the male will usally harrass her (or somtimes the other way around) because the females not in the mating condition.
how long have you had your fish?
and yes i copied this reply from your other thread
but the general flaring of the gills is a sign of agression since bettas are so teritorial. a little is ok but too much will cause him stress