CF in general is related to malnutrition...it could even be said to be malnurished from the lack of oxygen, etc., that the body needs to absorb from the lungs.
A lung infection, since P.A. is everywhere and a CF'rs arch-enemy, would be one thing to cause fatigue. Being tested more than once a year is highly recommended, at least for things like vitamin deficiencies, and lung infections.
Not having enough vitamins and minerals absorbed by the body would also result in exhaustion, or fatigue. Though a proper diet would also cause problems, too.
If the body is not given the proper diet, and the proper amount, the main result would be with gastro-intestinal problems. Putting it bluntly, if your stools are not being formed, it is one indication of not having proper nurishment. (took me a while to find that fact out.) Another factor in gastro-intestinal problems would be taking in too many fats, mainly animal fats, which is hard for the enzymes to digest. Since fats shouldn't, and really can't be avoided, trying to take in vegetable fats will at least increase the absorption factor, and the enzymes should be able to handle it a little easier.
In such a hard schedule of full-time school, and part-time work, along with your regularly scheduled program of life, it would be suggested to take the common vitamins and minerals suggested by the doctor, and trying to take in more garlic, since it is known to cut down fatigue. Also, Taurin is a supplement that also helps in boosting energy, and both of those combined may also help the body deal with stress, though the best way to deal with stress is by cutting it down! (yea, right! hehe)
Hope that helps out.