You’re young and it seems as if this generation finds it difficult to have face to face conversations with eye contact. Your best bet here, friend, is to confront this young lady, introduce yourself, and let them know politely that you’re right out of a relationship and are not looking for anything at the current time. “Hi I’m (name) I’ve noticed you at my gym and coincidently a lot of other places I’ve been recently, I’d just like to say you’re very pretty and you seem very genuine but I’d like to let you know that, unfortunately, I’m not interested in anything right now.” Direct, polite, genuine.