Hello, and welcome to the forums.
You said that you guys we're going well for about 5-6 months, and then with the start of school things are becoming different.
Maybe it is the stress from school starting again?
Maybe she's starting to think that she preferred you as "just a friend"?
I would say that the best way to find out what's up is simply to ask--you've been friends for years now, so it shouldn't be too difficult to bring up.
Hope everything works out in the end.
I will toss out that I wouldn't throw the friendship down the drain if in fact she's no longer interested in you as her boyfriend.
