My bf is best friends with a girl?

So heres the problem..my bf and I casually dated for almost a year and have recently been official for about 4 months. So obviously things are now a lot more serious and we are very happy together. However, his friends are over all the time including his very close friend who is a girl. Shes cool and we have always gotten along. She's basically one of those girls who always hangs out with the guys. Shes always very touchy and flirty with him and all his friends so I think shes just naturally that way, but recently its really been upsetting me. Only because she will be at his house typing a paper till 3 in the morning then stay the night in the same bed..and she leaves her blow dryer there, her slippers, and she buys her own food and drinks to keep there. Also, she will call my bf and he talks to her in front of my like shes his gf. It doesnt make me feel very special to be honest and on top of that she will come over all the time unexpected while him and I are hanging out. What do I do? Shes always so cool and sweet to me but at the same time I still havnt known her for very long, and who knows, she could be acting the same way with his other friends and not think anythings wong with it.