I've heard kegel exercises help. I think that's the name anyway. Just squeeze and release like you would if you were using the bathroom. Can be done while you sit or lie down and even if you're not alone, 'cause no one but you even knows you're doing it. I dunno how old you are, but I know that as women age, loss of bladder control is common. The kegel exercises are supposed to help as the freqent urination progresses to intermittent loss of control. I've got friends who developed that problem as time went by. But it's good you've gone to the doctor and ruled out other ailments. So if you don't have other symptoms like burning or itching, etc., maybe it's just the normal sign of getting older...
But like Daniel said, I'd have my blood sugar checked if I were you..
I used to have to pee more frequently when my celiac disease was undiagnosed - my body was in constant inflammation, including my bladder. If you've noticed other symptoms such as irritable bowel or skin rash then could be an inflammatory condition eg allergies
But 8 cups are not a lot of water, unless drinking that water constantly a little at a time and pee-ing a little at a time ? Maybe try drinking them in just 3 large goes and see if that results in 3 large goes - if you pardon the expression
There are so many possibilities,most quite trivial and some not so trivial that you should go to the doctor and get tested. If you are a wrinkly or have a tribe of children whre it is just part of life then it will be something that should be checked. Go see a quack
How many kids do you have? Pregnancy and childbirth can wreck all sorts of havoc on bladders. In general it seems like the more kids a woman has, the harder a time she has controlling her bladder, but I know a couple people who've had trouble after only one, so it varies some from woman to woman.
If that's the problem, doing Kegels is supposed to help. If you've never tried them before, try stopping the flow of urine mid-flow when you're peeing. Those are the muscles you need to work on. Squeeze and relax them periodically during the day to strengthen them. You're supposed to do it for 5-10 seconds at a time, 5-10 times in a row, and the sensation takes a little getting used to, so you might want to practice the first few times while you're alone.