Headaches, nausea, weakness, always thirsty, kidney pain, backpain: my blood shows nothing wrong , abdominal scan either but blood in urine.what can it be?

Hematuria is serious: It can be from small kidney stones. It can be from nephritis. If it persists you might need to see a specialist. It can be seen with urinary tract or bladder infection. You need an ultrasound of your kidneys and bladder with a full bladder. And perhaps a course of antibiotics. You need to discuss this with your doctor. You should not ignore blood in your urine.