Why Blood in urine? Cystoscopy showed healthy bladder; no kidney stones. Get UTIs but blood still there even with no uti! What else can be Investigated?

Thin GBM. One person in 100 has thin-GBM non-disease and lifelong hematuria. I bet this is you. If the proper workup shows no problems, you can stop worrying.
Reason for hematuria. Since you are on Coumadin for AF, it is possible that the bleeding is related to the use of this medication. You need to talk to your cardiologist. Hecan check your PT to be sure that your dose of Coumandin is correct, and he may offer a different medication. The urologist can tell you if there are other causes that need to be evaluated.