I forgot to mention I am 34, male.
Dark urine is a sign of dehydration or jaundice. If your skin and the whites of your eyes are normal, then you just need to drink more water.
the dark urine has to do with your kidneys. serious, continued diarrhea can cause dehydration which can cause your urine to be more concentrated. the fact that it is dark when you don't drink as much, and as long as you aren't having UTI symptoms, I'd say you just need to drink more fluids to keep your kidneys flushed. some meds can discolor your urine, tho. if you are concerned, ask for a urinalysis. talk to your doc about the changes in your bowel functions and see what he can do to help you out with that.
Have you tried cholestyramine? It worked wonders for me on that burning bowel fire and no-warning diarrhea post gall bladder surgery. It's a resin meant to bind with cholesterol, but it also binds bile salts and escorts them out if the body. Doesn't get absorbed systemically, which is nice. The only problem is it works a bit too well on me. I'm modifying my dose; The doc said 3 packs a day but found out I only need 1/2 a packet at bedtime, and only when when things are bad.