How should I be more approachable?

Tonight when I was at the bookstore, men left and right would wander and stare and bump into me here and there and lots of smiles and you know. I got in my car and gave a little scream because I am sick of being so unapproachable. I get really interested in doing my own thing and when I am in "the zone" i appear uninterested. I am also 5'10, I take really good care of myself, and I am always starting up conversation and smiling. I feel at ease in all social situations and am very confident and headstrong. In a relationship and when a guy talks to me, i am very comfortable with letting him take the reins and be in control. I just have had a wall up and have no idea how to be more approachable and have those random guys talk to me like they used to.