Should I be worried?

Alright ladies, this will be a long one. Been dating a guy for 3 years. Before our 3 year mark, everything was fine. He started a new job at the beginning of the year, a few months later he starts snapchatting one of the teenagers at his job. He assures me she has a boyfriend and nothing will happen. First message I see from her, shes bragging about a guy thinking shes pretty, to my boyfriend. So already I dont care for her. Then I find out, my boyfriend has been getting weed for her, she rode in his car on a few deliveries, AND she blocked me once she knew I had a problem with how close they are. My boyfriend told me none of this, found out from an old friend. They were snapchatting constantly, every time I came over. She started sending pictures of herself in the chat of snapchat and saving them! He still continues to tell me nothing is going on, despite me explaining why things seem odd. I dunno, things still feel fishy to me. Advice?