Paige, is it tired like no energy or more weakness? It would be nice if you could get some blood drawn when you feel that way to see what your potassium looks like.
I think the important thing is to be sure your doctor looks for other reasons for fatigue. For instance mononucleosis is very common in your age group.
Fatigue is certainly common with MC but if it's getting worse then I would suspect another cause.
A few years ago I started to feel really run down to where I would instantly want to get my pajamas on and rest some place warm after getting home, even if it was still light out. It turns out I was anemic and iron deficient. The doctor put me on some iron pills, which rectified the problem. You shouldn't assume this is the cause of your fatigue though, because it might not be, and it can be dangerous to take iron pills if you are not iron deficient. As Jan suggested, I would have some blood work done. Also have them check your thyroid.