Thanks for your query.
The blood in the stools may not be due to running.
There are many causes for Bleeding per rectum
.Most common cause of bright red stools is Haemorrhoids(Piles).If it is associated with loose stools,it may be Dysentry
due to Bacterial infection or Parasitic.
You didn't mention the drugs that you have taken after knee surgery
Any how,consult your Surgeon and get confirmed.
All the best