My lower back has been killing me for a few days now. My urine is fine, I drink water daily. What could it be?

Multiple reasons. Low back pain is a common problem and the cdc states that 80% of the population will have a back problem in their lifetime. It could be as simple as low back strain to sacroiliac dysfunction to other less benign causes. See your family dr. For a thorough workup and evaluation. There's no substitute for seeing, hearing, and examining a patient.