Pain in knee cap out of no where! it hurts when it's fully extended or while i'm walking. what could of brought this up? not doing anything different.

Out of the blue: Problems like this frequently come "out of the blue" meaning for no obvious reason maybe early chondromalacia of the patella try hot and cold packs both work well for non traumatic problems may also wrap knee with plastic wrap at bed time to limber it up and keep in body heat if not better in a few weeks then it needs attention of a doctor or more specfically an orthopedic surgeon.