Hi, Niccoli, and welcome! I'm so glad you found us and joined in. I usually have to use the bathroom about every 2 hours throughout the night. I'll have water with my dinner and then some water for my nighttime meds. I also have a glass of water in the bathroom and take a sip before heading back to bed at night. I do drink a lot of water throughout the day and I've never had a problem with dehydration.
I have found that if I have a small snack of something with salt, like potato chips, that helps retain the fluids and I'm not running quite as often at night.
I see you have mentioned this to your doctor. Has he checked you for diabetes? That causes frequent urination and I'm diabetic. You should be checked out to rule out other possible problems.
Be sure to check out the Fibro 101 thread....the second thread on the forum. There are links to good information about fibromyalgia. You will learn a lot. Also, there is a world of information in the back posts, too.
Don't hesitate to ask questions. We are here to help you. I hope to hear more from you soon.
Sherrine