You might be tightening the girth too quickly and pinching her skin and this is the only way she can tell you that it just hurt her. When saddling her up just fasten the girth up enough to hold the saddle in place without slipping if the horse shakes itself off. After you tighten it, pick up each foreleg and take hold at the forearm and cannon and stretch each leg so that any skin that may be pinched is smoothed out. Remember with young horses to gently and slowly adjust girths to prevent reactions like this becoming habit and causing problems later on in life where an inexperienced or unkind owner will punish this behaviour.