Why does girl friend tease relentlessly and then act not interested?

People only treat you the way you allow them too. If you do not like the way she treats you then you have to let her know and stand up for yourself. Communication is a very important part of a relationship and you must explain politely but firmly how you feel, what you would like and what is going to happen if this behaviour continues. It sounds as though she is playing games with you by teasing you for whatever her reasons then acting uninterested in you to have you chase after her and possibly even trying to control you but these are very childish games and have no room in a relationship.