How can I change how people perceive me?

I have recently realized that everyone thinks i am a stuck up snob. I perceive myself as a very friendly giving person, so this came as a shock. My good friend at work told me that other people at work and in college said things about me being a stuck up snob. She also told me that was also her first impression of me before she got to know me. Looking back, I have also had other people tell me the same thing, that it was their first impression before they got to know me. I am an attractive 29 year old nurse at the local hospital. I always try to dress nice, while everyone else wears sweatpants to work everyday- then we change into our scrubs. I work with the same 3 people every day, so im really not around my other coworkers except lunch and breaks. I am an outgoing person to the people I know, but, i am kind of shy and quiet when I don't know someone very well. I have always had a hard time making and keeping girlfriends. I have been making an extra effort to talk to people more and be more friendly. What else can I do to change peoples image of me. Why do you think people are getting this impression of me?