Sneaky?

My boyfriend and I will be together for a year in January and are planning to move out together in the beginning of the year. My boyfriend has just started taking one of his middle school friends to work. She either has to walk for 45 minutes to get home or get a ride that'll take 5 minutes. One day while my boyfriend and me both had the day off and she had to work and needed a ride home. We already have been on a double date with her and her boyfriend to the fair. If we already hung out with each other at the fair why would it matter? I talked to my boyfriend last night ams told him how I felt. I even called her a sneaky bitch. (After all she is acting like one) He asked me if I trusted him and I said that I do and that I don't trust her. He told me one day the brand of makeup she used. I then asked him what brand I used and was only able to tell me one that I use and not all of them. Should I be worried? What should I do?