Could be a UTI. I have one. Didn't even know I did either. But they're more common than not in a pregnancy.
It may be blood. Go to your doctor, it sounds like a urinary tract infection (uti) or kidney infection.
Your body will flush out the extra vitamins and minerals in your urine that your body doesn't need or use. Its just part of the functions in the kidneys (waste). So my guess (if youre not having other symptoms leading to diagnosis of infection) is your dark urine is an overload of vitamins and minerals I'm which your body is excreting extra waste tgrough kidneys. Extra vites probably from prenatals if you are taking those.
Your prenatals will cause your pee to be school-bus yellow. I think it's the Vitamin B.
Mine gets that way when i have a uti or a bladder infection. I would defiantly keep calling your doctor and bringing it up. Its simple to test for it, the make you pee in a cup and test your urine and if you do have one, they give you antibiotics.
You should be shooting for drinking half your weight in ounces a day. So if you weight 100 pounds you should drink 50 oz a day. If you are drinking tons of water, then it could be a kidney issue. I would make your doctor listen to your concerns!