Why is my bf acting like this?

(repost) K.. At first i thought it was kinda sweet that my bf insisted on going to the bar for me, walking me to and from the toilets ect.. but when we were at a small house party the other night he was flirting with other girls and using me to make them jealous =( (putting his arms around me a smilling at them) why does he follow me around as if i am going to cheat if he is the one who keeps flirting with other girls? i wouldnt mind a little flirting but not when he was doing it so much.. should i walk away? thank you^^