I would check with the vets, but I don't think it will be anything to worry about.
With upset tums, especially those prone to be runny tums, it often happens that the bowel is irritated so you do get a bit of blood, and sometimes mucas as well.
That's the case in hoomans, anyway, not sure if the same applies to our furry friends, but defo check with the vets on the phone as you're concerned about it.