She said she has a bf but she acts interested, doesn't she? Was she lying because of my "disinterest" (long)?

Alright, first confirm if she is keeping people away from you. Do this by bringing a friend to the gym. Be friendly with that friend, and then tell you friend to stay in the gym for another hour while you leave. Keep your friend informed on the plan, and if she is rude to him/her, then you have your answer, Confront her the next day that the guy was your friend, and ask why she was rude to him. Shes going to hesitate at this point. Then ask her if she likes you or something. Due to the pressure, she will confess.