I get blood in urine after having an erection. What should I do?

See a Urologist. Gross blood or blood you can see in urine should be evaluated by a urologist. It may require some urine tests, ct scan imaging and possibly a cystoscopy (or direct look in the bladder with a small scope). Questions to consider: any pain with blood, any associated symptoms, are you a smoker or not, any other voiding (peeing) issues? See your pcp for a referral.
Hematuria. Hope you had no trauma or injury. If no then at age 63 possibilities are prostate infection, urinary tract infection , hemorrhagic cystitis or simple stone. Is it painful or painless blood in urine. Please see urologist. Wish you good health.
Abnormal. And needs to be checked out by your doctor and possibly a urologist, .
See your doctor. The two most obvious reasons are an infection in your bladder or prostate or trauma. Either way, you ought to see your doctor and have this checked out because this is not what usually happens after an erection.