Should I carry on with my relationship?

My girlfriend and I have been romantically involved in love (no sex however) with the prospect of marriage. It's been 2 years since I first confessed to her. It's really good in the early stage but since the last few months, she's starting to change very much. She once confessed to me that she did not want to be involved in love until she obtains her degree and I respect her decision. However, we still are in a relationship albeit that confession of her. When she is mad, she would say that that's why she didn't want to be in a relationship until she has her degree but when she is in a happy state with me, she would call me 'sweetheart' or anything of that kind. But starting from a few last weeks, she became so cold towards me, both on Facebook messages and phone messages. I would ask a very long question and she would reply a very disturbing kind of reply such as 'okay then', 'don't know' although she clearly knew the answer. When I checked her Facebook messages, I realized something. She had a guy friend she messaged a lot and she treated him way better than she treated me, such as giving smileys etc. I don't really think she's cheating on me because of the lack of intimacy between them but I'm depressed when she treated me (her boyfrind) unfairly unlike when she treated that boy who she just knew for a few months. I might be too jealous but it disturbs me so much about that that my studies are falling apart although my exam is just a week's away. I had confronted her about it and she gave me a very stupid answer saying that the boy happened to be the friend of one of her guy friends (her guy friend's friend) and she had to treat him nicely. I was like 'what the hell?'. She treated the stranger even better than she treated me. What should I do? Please help.