Just some comments on James' comments: Yes, do your research! Being fully informed is very important. On the other hand, self-diagnosis can be risky, even for a physician who has the knowledge and education. There is a ton of spurious and inaccurate information out there on the internet (the modern equivalent of snake oil and Lydia Pinkham), so be careful to stick with the more reputable sources.
If the suggestions given here, especially from Jennifer, don't work: Consider a physical therapist. Or look for a sports medicine specialist. You may need both. The PT would be the cheaper option. Sometimes delaying treatment can make things worse.
What I've learned from my own problems: Once there has been an injury, especially one involving any joint or ligaments or the spine, it's vitally important to keep the supporting muscles strengthened. As suggested, shoe changes may help. Trekking poles can also be extremely helpful on those long downhills.
I had a severe knee injury (skiing) 25 years ago this month--I basically ripped all the ligaments and had to have them replaced (with pieces of patellar tendon, the only one that didn't get torn). That of course means my patellar tendon is also weaker. Any time I let my knee exercises (including using a bicycle) go, it starts hurting. I also have to be careful of overuse injuries, especially when training, working up very gradually when increasing exercise levels.
I do still wear a neoprene brace for hours-long steep downhill stretches with a pack. I try not to wear it otherwise, though, because constant use of the brace weakens the muscles that I need to be stronger.