What are some possible reasons for blood in urine, pain in lower back, general fatigue, fainting? I only have one kidney

The. The most likely explanations are infection or a kidney stone. With only one kidney, you should not take any chances: I would recommend that you see your doctor and have a urinalysis.
Call doc now. Since you only have one kidney, you really need to be proactive about urinary tract symptoms. A urinary tract infection, kidney stones or inflammation can cause these symptoms. You need to been seen today (urgent care if open or er) or at least talk with an on call doctor for the practice you use.