My dad just had his catheter removed yesterday and the nurse at the appointment said that sometimes if you do too much (lifting, etc) there might be some blood in the urine. She said that is why they don't want them to be lifting more than 5lbs (we were told ten originally, but yesterday the nurse said five) for 6 weeks following surgery. Obviously, I am not a doctor and there could be a number of reasons. Good luck!
Another doting daughter :)