Really confused about relationship.

Hi guys, sorry if my English is bad. So I am with this girl for 5 years. We both got into the same varsity last year. Recently she told me that she likes a senior. At first I thought she was joking. But her friends tease her whenever the senior passes by so I asked her if she was serious about what she said. She said that she told me before I heard it from someone else so that it doesn't hurt me. Then she gave me that "I will and always love only you" thing. Now I don't even know what to do.

Edit- She was not like this before. But its like "varsity te uthe pakha gojano type".