Sorry, I wrote a response earlier, but for some reason it never showed up.
A little blood isn’t all that unusual, especially after having a bowel movement or a bladder spasm. Normally it’s either irritation from the catheter or bleeding from the surgery site itself. Sometimes a small amount of urine is actually mixed in as well, making it appear as though there’s more than there is. You can put some Vaseline or KY jelly on the tip of your penis to help lubricate and protect it.
I would call the doctor’s office and speak with a nurse, just to let them know and double check. None of us are actually doctors as far as I know, we just pretend 🙂