I get confused for a doc all the time. Maybe it's because I'm 36, roguishly handsome, and walk with confidence... because I'm a damn fine ER tech.
Maybe it's your bearing, posture, diction, or some other Dale Carnegie-style issue of personal presentation. Are you an attending? Resident? What year? How old are you? Does your program offer any training (official or unofficial) in stuff like how to walk into a room, how to project calm confidence and sympathetic authority, or do they just leave you to figure it out? Do you have any mentor-types who are never confused for anything else, whom you can emulate?
Heck, maybe it's just that there isn't enough differentiation in the uniforms. Need more info to form a hypothesis, really.