A Few Times I Urinate After Playing Basketball It's Bloody.

A few times i urinate after playing basketball it's bloody. I went to the hospital yesterday and they said I had some calcium oxidate or something in my urine sample. I've had a kidney stone once before and the same thing was happening, I would urinate and there would be blood in it. But this time I have no pain in my lower back or kidneys so I don't think it's another kidney stone. What do you think could be causing this bloody urine?