Alison is spot-on about parents.
You may want to set yours down and tell them that it is offensive and silly for them to exaggerate your gender-issues or to seek advice regarding them from salespeople. They ought to take their concerns to you and the psychologist they ought to provide for you.
As for the fitting-room guy, forget him. You cannot tell if you pass by walking unchallenged into men's bathrooms or men's fitting rooms. The vast majority of the time, nobody will say boo even if a cisgendered girl dressed as a girl does that. Obviously you didn't pass, but if you are passing you might pass all day and not know it, because you can't be sure until people call you 'sir' or the like. Anyway, probably the fitting-room attendant decided that since you're young, he could pick on you and get a little satisfaction from being King of the Fitting Rooms.
On a similar note, men's restrooms -- just watch the men around. If you pass for male but use the ladies, you will get dirty looks and possibly shrieks. This is nothing compared to what might happen if you don't pass and use a men's room in the wrong place. It's actually highly unlikely that anybody in a men's room will even look at you, no matter what you do, but the consequences could be grave. Nobody has ever noticed me using the men's room, but I don't use it if I would be worried to walk down an empty street with the guys around said men's room behind me, or if they seem to be ultra-conservative types who'd likely make a fuss. Most places, females, even obvious ones, using men's rooms is a non-issue that will come to naught, though most FtMs find it a big step and a psychological milestone.
