I Am 67 Years Old (cau Male) And Consider Myself

I am 67 years old (cau male) and consider myself to be in very good physical condition. I do a very rigorous cardio workout at least 2 times a week with other types of exercises (not so rigorous) in between. Today being Saturday my last rigorous exercise day was Thursday. Yesterday, I did some lifting of heavy boxes. Today, I am experiencing blood in my urine when I pee. No pain and I still feel great. What may have caused the blood in the urine and what should I do about it. Thank you!!!