If u mean it holds its postiion almost vertically in the tank, then i think that is normal, nearly all tiger barbs tend to do that (at least all the ones i've seen ) But if u mean acctually upside down, then it sounds like a swim bladder problem.
just to clear up. there is an upside down cat that swims upside down at the top of the tank. takes it's food that way. doesn't like to swim right side up for the most part.