boyfriend's birthday

yesterday was my boyfriends birthday and it was the first birthday we've been together... well, he only has one friend, he really talks to and she a female... well, they were going to go out to dinner and he invited me to go, i said if he wanted me to go, id go, but i don't want to pry... well, his birthday has came and he ends up telling me he's just gonna go over there and watch the movie i bought him for his birthday with her (I'm not too big on the same movies as him, but will watch them for him, and same with him for me), and they ordered food in i think... well, i know about 5 years ago this girl came on to him when they first met... and nothing happened, except maybe a kiss.. but it was 5 years ago... but I've never met thus girl.. they've went out to the movies and a drink before since we've been together.. but that was more early om in our relationship, now i have more feelings and everything... and idk if i should be worried since he went to her house ? i haven't had any reason to not trust him, and we're usually always together except while at work...