He used to like me and now he ignores me?

theres this guy, he used to flirt with me ALOT in class and he asked for my number...but he has a gf. anyways he used to flirt soo much but then after class when i used to see him in the hallway he totally blanked me. then in class he'd flirt with me again. however, after the 2 weeks holidays he just totally started blanking me. then yesterday in after our exam he was like blanking me deliberately. and today i saw him and he didnt even say hi and was definately blanking me but sometimes i could see him looking at me quickly from the corner of my eyes and he doesnt even act normal around me, he treats me as if he never even knew me :s im so confused.....