Hitting???

Why is my little girl hitting other kids?  When she gets frustrated at home she will hit me (which I try to explain its naughty) but with other kids she is playing then all of a sudden she just hits them for no apparent reason.  I always pull her away tell her its naughty to hit and let her stand/sit for a minute but doesn't seem to sink in.  I do hope she gets out of it soon as I don't want her to be a 'bully'....