Balas get spooked really easily, it helps to keep them in groups of 3 plus but as they get around 13 inches long you would need a pretty big tank for this.
Bala sharks can't cope being in tanks too small for them and they also need to be kept in groups of at least 3, they are difficult fish to keep since they require a large tank to live in, at least 75gals usualy, and are very skittish fish so all tank ornements/rocks that they could hurt themselves on need to be removed.
You probably just scare the fish some how. I have 3 before. They got nutz when ever I go near the tank. Pretty normal to me. Your tank probably located on bussy trafic place. Try to put more plant/deco for hiding spot so she wouldn't freak out as much as before also a company of 1or2 would be great.