Blood in urine, but urinalysis shows no infection?

[Tekkened]

I am a 21 year old male. So I had some serious lower stomach pain. It hurt so bad I thought I was having appendicitis. So I went to the emergency room and they tested my urine and did a ct scan. They said that the ct scan was normal but they did find blood in my urine and they suggested referred me to go see a doc in a week and sent me on my way. So I went to see the doc and they did a urinalysis and they said there was white blood cells in my urine but no bacteria grew so that meant that there wasn't an infection. We tested my urine again in 2 weeks and there was less blood but the blood was still there with no infection.

What I am trying to figure out is why there is still blood in my urine if there is no infection. The doctors said that they think it was just kidney stones because after I left the emergency room it did hurt at the end of me going pee. But if it was kidney stones I feel like they would have shown up on a ct scan and I also feel like the blood would be gone by now since I passed it. If any of you can help me understand this that would be great, Thanks