My 14 year old daughter has constant knee problems for about 5 years now. What could this be?

Growing pains? 5 years is a long time for a bruise or other traumatic issue with a teenager. It could be growing pains. I would try a regular/scheduled dose of Motrin or Aleve (naproxen) for a couple of weeks, but if not better, she should be seen. There are conditions, such as osgood-schlatter disease, that would manifest in teens.