I believe HA and somatoform to be closely related. If you look at the HA forum you will see people are more or less convinced that they have a certain illness. Some are even surprised that their symptoms get worse when their anxiety increases... as if the two aren't directly related.
Regarding your fatigue, I suffer from Chronic Fatigue Syndrome. It is essentially just a worse form of the fatigue you experience. It has a name, but it's not in the least bit understood. I am currently doing a programme for recovery called Gupta Amygdala Retraining. In this programme we essentially learn to confront all of the negativities and anxieties in our lives, both surrounding the illness and also in general.
You would be amazed at the physiological results people get from essentially psychological methods. People with fibromyalgia who have been in chronic pain for years return to normal. People with multiple chemical sensitivities once again become immune to the odour of cigarettes and carpets. I just read an account of someone who had CFS for 25 years - most of which was spent in bed - who the other day ran a 10km run. This is all due to correcting the adrenal cycle and the sensitisation of the sympathetic nervous system (the fight or flight response), which has a domino effect on our immune systems, our nervous system, and our GI tract (very important to wellbeing).
I get you that it is so difficult to see that psychological issues can cause such dire physiological symptoms. I really do understand that. However, our mentality is so deeply tied to our physiology that it is impossible for you to live with worry and not have serious effects. Let me give you a few examples of how the mind can effect the body (and vice versa).
1. In a recent study they found that certain social or psychological triggers can activate an immune response. This could be a certain perceived contamination, for instance, that has cause an immune response in the past, actively retriggering an immune response in leu of any actual pathogen. (e.g. I got ill when I ate this last time, thus my immune system is triggering this time as well).
2. The placebo effect - the act of administering a fake treatment for a real illness with positive results - is widely dismissed as 'oh, it was just the placebo effect'. However, the placebo effect is capable of actually causing obvious physiological changes. For instance, if you take two groups of people with any given treatable virus. The first group is given a placebo, and the second is told just to get bed rest or whatever. Studies have shown that the first group will clear the virus quicker, with immune markers such as inflammatory cytokines reducing to normal levels faster than the non-placebo group. There is nothing occurring here other than a belief in getting better, though it has very, very real and measurable impacts.
3. Studies of cancer patients who had a positive outlook showed they had betters chances of survival.
4. Stress causes a degradation of the intestinal lining leading to a number of illnesses. Moreover, it can cause ulcers (as you know very well).
5. Chronic stress obscures the immune system from a balanced Th1/Th2 response to a predominantly Th2 response. As a result you are less able to fight infection, however you are more prone to allergies than someone with a balanced Th1/Th2 response.
6. Looking at it from the other direction, the administration of regular probiotics has been shown in rats to significantly reduce levels of anxiety. This is because the GI tract is part of something called the enteric nervous system that is attached to the brain via the vagus nerve.
So, as I hope you will agree, this very short list of all the possible impacts of psychological elements on our physiology demonstrates the very real possibility that if no physical illness can be found it is very, very likely that your continued state of sympathetic nervous system arousal is leading to a degradation of your physiology and is likely causing you fatigue.