When you know somethings up with your girlfriend?

My girlfriend and me have been going out for almost a year and i love her to pieces and she knows that but lately shes been down. I mean crying, tearful and isolated. What could be wrong with her? I try to give her a hug but shes crying and i try to kiss her but she seems blank with teary eyes. Ive asked her if we should split up if that is the problem and she said no. I asked if someone died and she said no. I dont know why she is like this? I care for her loads but i feel bad when theres nothing i can do. Anyone know whats up with her or how i can help.. Please! i really need some advice. Both 18 aswell :/ Its really confusing and heartbreaking because i cant do anything when i havent got a clue whats wrong with her.