Didn't pick one because its a matter of situration. For me personally if the dressing room has curtains instead of a door then I don't like men back there because those curtains never seem to close properly and always leave slits. I don't know the guys so I don't know if they are trustworthy or not. If it has doors then no problem. I usually have my husband stand near the dressing room so I can pop out really quick to ask him about something without actually inviting him in.

Now that said no one was in the dressing room area, doors or curtains aside, so you had every right to be in there. However the lady also had every right to ask you to leave if she didn't feel comfortable with you there.
I don't know what her tone was but if it was rude or angry she was wrong to use the tone.