Identifying the source of some pain

I hope I've posted in the correct forum. If not, if an admin could move this I'd appreciate it.

I've experienced two kinds of pain recently on overnight trips. So far it hasn't been a real issue because I've only been out for one night. However, I'm planning longer trips and would not like to find myself incapacitated on the 3rd day.

The first is noticeable only when I'm going downhill. It is a pain in the back of my knee and in the front, under the knee cap, towards the bottom. It's a sharp pain and at one point reduced me to walking with my leg held rigidly straight in order to avoid it.

The second is a pain in the back of my heel. I thought it was a blister beginning but it's not. It feels almost like a stinging. When I press directly on the spot it doesn't hurt, but if I press and then push my finger up or down while holding the pressure I can feel it.

Has anyone had experience with either of these issues? I typically hike in a pair of Salomon XA Pro 3D Ultra's. Total skin-out weight is generally between 25-35 pounds (I'm still working on it!). My muscles seem fairly well conditioned as I never experience fatigue while hiking and have only minor soreness. My theory is that my typical aerobic exercise (stationary bike) does not put enough stress on my joints and that backpacking is then a bit of a shock.

Any thoughts/suggestions?

-Dave