should this happen between me and my friend?

Hi im a 15 year old girl and i have a friend who is a 20 year old women. Me and are really close and we see eachother all the time. When its time for me to leave we will always hug eachother and kiss eachother the cheek then we started kissing on the lips. Im not gay and she isnt either but she always gets rid of her bf and makesure no one is around when we do that and i think thats strange and im comfused. Please give me your thoughts and be nice