Are you me bro? I had this exact thing happen to me. I'd been seeing this girl for around six months. When we first met up she'd only recently broken up with someone so I wanted to wait to ask her out so I could find out if it was just a rebound for her. Months went by and we'd been talking frequently and met up several more times. Would have met up more but the distance between ourselves made it a little difficult. So, needless to say I was really infatuated with this girl. I thought that I'd be able to ask her out and get my first relationship. Then, in January of this year I saw her facebook feed update with a "In a relationship".
My advice would be to leave her be. She wasn't honest with you about the other guy. Who's to say that if you were dating she wouldn't hide the fact that she was with you and start seeing other guys as well.