How to talk to a friend about some things?

She and I have been best friends since 1st grade and are very inseparable. We consider each other as sisters. But lately its been harder to talk to her about certain things. She and I are now in College and going to the same college, just different majors. Ok at the college you are supposed to register for the next quarter of cles on the 7th week. My friend had to wait til Dec 15 to register. I got mine registered on the 7th week. She didn't make her biology score of a B- but got a c+. She is now making excuses on everything. Like if I want to take a walk with her or about her cl schedule. Some of her excuses are that it is too hot/cold/wet/ or dry to take a walk or that her tuition money won't pay for any other cl but the one she got recently. Her grandparents and I have gone around and around about it. I am now walking on eggshells with her because I have 12 credit hours of cles and a 3.9 GPA. So how do I talk to her??? I am sick of her excuses but I don't want to abandon her. She is my sister through and through.