If you have real suspicions, then calmly approach the subject, explain you are feeling insecure about the closeness she has with your best friend. If you stay calm and don't accuse you will get your answers from the way she reacts and speaks back to you, getting angry is not the way to go....unless she confesses of course! You may just find she is being friendly because she knows you are good friends and she wants to be part of that for you, she may not realise this has become a problem for you.