I like this girl somtimes who likes me also, but when im with her shes different, Im not sure what to do?

I have know this girl Mary for over a year now, im 16 and she is 15 shes extremely shy, but we became good friends despite her being kinda weird, however when we are on the phone or texting shes really cool and very sweet, but the next day at school shes different and kinda weird again, even when where alone. She has told me she has feelings for me, we get close on the phone but, when where together, I reconsider my thoughts about her, im not sure what to do?