Sounds about liek a single lunh hit.You should of waited before trailing it.So many people make marginal shots and dont wait.Unless you see that you hit it in the lungs or heart then you should of waited longer.Atleast 2 hours at minimum.I will venture to say that the elk lived.Especially if it was only a single lung.They have a pretty good chance of living with only one lung.I have had a buck turn on me while shooting fortunately for me I still made a good hit and it went only 30 yards before going down.Things happen.I would say the elk knew something was wrong and she was skittish and that is why she turned to leave.Prolly almost ducked your arrow in a matter of speaking.
Better luck next time.