\\Could I have done something nasty? \\
Extremely unlikely.
\\\Can kneecaps split/become damaged etc? \\\
Yes, they can be fractured, dislocated and have ligamentous tears, but in your case it is again, extremely unlikely as you can walk without pain.
\\\You might have spurs, which is very painful.\\\
That is a possibility, but unlikely. The likeliest cause of your problem would seem to be a pre-patella bursitis (Housemaid's knee) and providing it doesn't become infected (red, painful swelling) it should settle without further treatment.