Well, first of all, welcome to the boards!
Chronic diarrhea with no known underlying cause (termed "idiopathic") is very common. From your use of the word "consultant", I am guessing you are from the UK, AUS or NZ?
In the U.S.A. idiopathic diarrhea usually ends up with a diagnosis of "Irritable Bowel Syndrome" (IBS for short).
Did your consultant call it that? IBS drives doctors mad because they can't find a definitive cause. They are SO trained to diagnose, treat and/or cure, so it's very frustrating for both doc and patient.
I presume your doctors have done blood tests?
Have you ever had a colonoscopy?
A barium enema?
Is your diarrhea painless?
How often do you have to go?
Do you ever have "good days"?
Have you been tested for intestinal parasites?
Have you ever tried an elimination diet to ascertain whether you are sensitive to certain foods?
Gluten and milk products are common culprits.
What meds have you tried so far?
Without knowing more about your health history or what tests they've done for you, on the surface, chronic diarrhea can cause you to become dehydrated
(even if you don't actually feel thirst) which alone
can cause fatigue, lethargy, lightheadedness/dizziness.
Diarrhea can cause salt and potassium depletion
which can also produce the same exhaustion.
As for your joint pain, have you ever been referred to a rheumatologist to find out if its arthritis of some kind?.... and as for the aching, if the pain is a chronic
all-over-muscular-type pain, it may be fibromyalgia, for which there are medications to help.
The back pain could be related to your general aches and pains but impossible to diagnose on the internet. If the back pain is acute and really interfering with your life, here's where an orthopedist might come in.
Your back pain could simply be the result of not enough exercise, and when you feel so lousy, I'm guessing you're not getting much of it lately!
Sorry to bombard you with so many questions, but the more info we have, the more we can help (I hope!) or at least steer you in some direction, or arm you with specific questions to ask your doctor.