Possibly it is kidney stones that have been broken up? If it keeps up over a week you may want to make a doctor's appointment.

A “dark brown” is old blood. Not fresh blood. I got that from bladder stones. Yes, bladder stones, not kidney stones. Bladders stones occur when you have an enlarged prostate and cannot void all of your urine, so, urine will accumulate in the bottom of your bladder and the uric acid forms stones. Both your inability to void all your urine, and your bladder stones will get worse if you don’t see a physician/urologist. I eventually began to void very red urine because at the time I was a jogger and the stones bounced against the wall of the bladder causing it to bleed. They stick a sonic gun up the urethra and blast them into little pieces. This requires hospitalization because you also have an enlarged prostate and they will also do a TURP (trans urethral resection of the prostate). It requires 2-3 days in the hospital, and hurts like hell for a while, but you have to have it done to get things back to normal.