My cat gave birth to 5 kittens 2 days ago, her tummy is till hard and swollen and she has started bleeding. Is this normal?

How is she acting? Have you taken her temperature? How is her appetite? And is she nursing and caring for the kittens? A bit more information would be helpful.
Also did you happen to have an x-ray done before the birth to see how many kittens she would have?
Does the "blood" have a foul odor or strange color?
It may be time for a postpartem check-up, just to be on the safe side.
Cats have been having kittens own their own for centurys, but just like us they too can have problems.

Yes, it does sound like she has retained a kitten, but without an exam it is hard to tell for certain.

Having delivered 5 kittens without incident, why has she suddenly, completely stopped?

A dead kitten doesn't deliver easily, an exhausted uterus doesn't deliver easily, a damaged uterus doesn't deliver... period. Can anyone be certain that the uterus is still intact, or may it have ruptured, while trying to deliver the last (?) kitten, and, possibly, even delivered the kitten into the abdominal cavity through the tear? That may be quite a leap, but if so, it needs to be addressed much sooner than waiting a few more days to 'see what happens'. Often, vets are forced to rule out the worst case scenarios before attacking more likely, lesser serious problems.

All around best suggestion is go to the vet and let him/her decide. Best wishes.