Hi,
There are a number of possible causes.
1. Joint problems
- these include patellofemoral joint dysfunction (but why is it sore?), mensicii damage (but why did it get overloaded?), Ligament or tendon overuse (why did it fail and not the other side?), fracture etc
2. Muscular system problems
- these include tears, strains, cramps, hypertonicity, etc but it is hard to guess from the above info
3. Neural system problems
- these include motor control patterning issues, nerve entrapment (saphenous nerve is a distinct possibility here), referred pain from back etc
4. Any blood flow issues - not likely to be honest
5. psychosocial reasons - probably not likely either.
If you can sort out those areas with a good physical therapist, then you are on the way to working out what is wrong with you...
Sorry i can be of more help
Cheers