Why does my best guy friend make me so nervous?

I admit i like him (i think he likes me) but i havent been this nervous around him before. when we talk i smile alot and i think i blush. we will be walking around and he will walk closer then usuall and sometimes i feel him brush his hand against mine (i am not sure if its on purpose or accident but he didnt use to do it) and one time he put his arm around me and said "hey" i said "hi" back but its so unusual for him to act this way why does he? and why do i get so nervous? please help i might see him today.