Why is my mouth so dry?

It's been dry ever since I went to work today around 6pm. I had to go to the back and drink like two cups of water then when I went back to work my mouth got really dry again. I asked if i could have a bottle of water with me (I'm a part time cashier, so obviously I have to talk a lot too) and they said it was fine as long as I didn't drink it while I was serving a customer. I downed a whole water bottle and still no change. It's 4 am and I've had two glasses of water and a can of pop. My mouth is STILL dry! What in the world?!