Yup, I would look into HPA-axis dysfunction, also known as "adrenal fatigue" in layman's terms. What may be happening is that your body is under so much stress (life stress plus post-transplant stress) that your ability to produce cortisol (which is the hormone that wakes you up in the morning and keeps you awake, mediates stress responses, and also has many other effects on the body, such as inflammation suppression) is all but nonexistent. Another sign of this may be out of control chronic inflammation, but since that's a CF thing in general, it may be hard to tell what's a result of adrenal fatigue (lack of cortisol) and what is the background level of CF inflammation. One thing you may be able to do is get a saliva test of your cortisol production throughout the day. Regular MDs usually do not order this test, so you may have to go to a naturopath or osteopath to get this done. You essentially spit out a sample every few hours for 12 or 24 hrs to measure the level of cortisol (and maybe some there hormones too) you are producing throughout the day. If our overall production is low, there are several things you can do to help. I would recommend listening to this podcast on the basics of adrenal fatigue and what to do about it:
http://chriskresser.com/ask-the-rd-adrenal-fatigue-2 . Be aware that if you're on any immunosuppressing anti-rejection drugs, that you may need to look into the interactions of certain herbs that are indicated for adrenal fatigue, particularly string ones like licorice. Working with a qualified herbalist or naturopath may be important for you in this regard.
If your cortisol production seems to be fine (highly unlikely but possible), then I would look into thyroid dysfunction. Has your doc tested you recently for hypothyroidism? That should certainly be tested for and ruled out as a possible cause. Hypothyroidism causes tiredness, brain fog, fatigue, hair loss, etc. Even if your thyroid levels test normal, you may still be at risk for hypothyroidism since the underlying cause (autoimmune - Hashimoto's thyroiditis) may be present. I would refer you to Chris Kresser's website for more info on this. He has a whole free eBook on it:
http://my.chriskresser.com/ebook/thyroid-disorders/ . There is also a good introductory article I came across on this topic:
http://hypothyroidmom.com/hashimotos-your-body-is-not-supposed-to-destroy-itself-right/
According to the last article, one way to test for hypothyroidism is to get the obvious TSH tests done (the ones your docs do regularly) but in addition to get our thyroid antibodies done:
Thyroid Peroxidase Antibodies (TPOAb) and Thyroglobulin Antibodies (TgAb). These are simple blood tests.
Also, I've been struggling with low energy also, and I recently started to take CoQ10 (100 mg once a day) and this seems to have really helped me. CoQ10 is a molecule needed for mitochondrial production of ATP, which is the main energy source for all of our cells in our body.
Even though you can't sleep all day, you are getting at least 8 hrs of sleep a night I hope? If not, that's of course #1 on the list of important things to do. There is no replacement for sleep - it is absolutely critical for health, even a mainstream MD will recognize that.
Hope some of this helps. 
