The kneecap "protects" a lot of sensitive ligaments and features, such as the ACL, PCL and meniscus. The MCL runs along the "inside" of your knee. You could have damaged any of these, some of these, or none of these. I at first suspected aggravation of the patellar tendon, as aggressive downhill movement places an unusually high load on this area of the knee. Yet your description of pain "inside" the knee does not support this; indeed, it is indicative of the previously mentioned features.
Since you were hiking I presume there was a lot of quick movements and changes of direction, which would certainly, to an extent, endanger the precious ligaments of the knee area. However, in a regularly athletic person, it is rare to see trauma without the individual sustaining injuries to the region previously, OR without a more specific incident. I.e., rather than 'running down a hill', something more along the lines of 'running down a hill and twisting my leg and immediately feeling a strange sensation... etc'.
It could be as simple as bruised bones, but this would only result if you landed with our leg at or very near to lockout, again you would have noticed this when it happened, unless you were boulder hopping >10 feet, less with a backpack. If you wish to have a more specific hypothesis, more information is required, rather than the general issue statement of possible knee injuries I have issued. For instance, be more clear on the pain and what aggravates it: twisting the knee outwards or inwards; moving the knee through its natural ROM; can you palpitate anywhere and aggravate the pain; can you load the knee (2 legged squat? 1 legged squat?).... et cetera.
What most PTs and Dr.s will do first is a series of stretches/movements to figure out which ligament/feature is likely damaged. You can simulate these at home, but it helps to have someone else do it. I am not necessarily advocating self diagnosis. Anyways, with a more specific account of what happened on your downhill assault and what the pain is like now, we can probably give a better guess.
What you should be doing regardless: RICE, plus ibuprofen. And no running/leg workouts. Hope this was helpful.