She's drinking more water but urinalysis normal

For just over a week now my dog has been taking on at least three times as much water as she usually does. Her water consumption still is within the "normal" amount for a dog her weight (28lbs) She gets through between 450ml and 600ml a day. But she is now drinking 5 times a day instead of once or twice as before.
Otherwise, her general health and fitness is excellent for a dog her age. Her appetite is good, she runs, plays, walks about 5 miles a day, sleeps well etc. and seems perfectly happy.
But even though I walk her twice during the evening (once last thing before bed) she has now wet the bed twice this week. I don't mind that so much. It's manageable, except it shows something unusual going on.

I took her for a urinalysis. All her values were normal. No sign of kidney or bladder issues, no sign of imbalances of any kind, and no sugar diabetes or protein. Bilirubin also OK. Also her Urine Specific Gravity was normal. This is the report my vet gave me yesterday.

I am at a loss now to know what could be going on with her?

He told me to wait two to three weeks and if she was still the same, they will do a blood test. But even he can't explain why everything tests normal.
I haven't restricted her water. If she needs it, it's there.

[Aleda M Cheng, D.V.M., C.V.A]

A culture and sensitivity of the urine and a urine cortisol creatinine ratio may help to rule out a subtle urinary tract infection or as a screening test for Cushings Syndrome.  

A blood test is necessary to determine if there is full blown kidney disease.   However, once kidney disease shows up in the bloodwork it means that 75% of kidney function is already gone.  I hope that your dog doesn't have it, and probably doesn't have kidney disease if the specific gravity was normal and there was no protein in the urinalysis.

A therapeutic trial with an antibiotic that treats urinary tract infections might also be appropriate.

[ginger899]

Thank you Dr Cheng. I do appreciate your help.
I will get the vet to run those checks. He did mention that there were no signs to make him suspect kidney disease or any loss of kidney function at all. He did mention that there were no signs present to indicate any urinary infection even slight.....
But the cortisol/creatinine test he didn't mention...I will speak to him.