my betta does that once in a while, one side just flares, but sometimes both does... hopefully its not a problem... I just moved him from his 1 gallon to a 2 gallon (all I can afford) and he seems completely happy w/ it... he flares at his own reflection and he seems to be ok.
im not an expert but maybe one of his flairy dealies (laymens terms?) doesnt work. is he acting normal? like does he still swim around fine and eat okay and stuff? if so i wouldnt really worry about it, i think it should be fine. im no expert so id definatly let some other people comment on it to see what they think.