Knee Pain. What do i do?

I usually run about 30 miles a week but this week on sunday, after my 4 mile run, i started to get this pain in my left knee. It mostly hurts when more pressure is on the leg (ie- walking up and down stairs) but also even sometimes when just walking or sitting down. im not sure what this pain is (i was told it could be something to do with my tendons.I also still want to run, im not used to taking long breaks from working out and next week ill be gone for  5 days so i wanted to get more runs in this week.

So what should i do about this pain and what exactly is it?

If any one has any ideas, please let me know.

Repetitive stress injury? I highly recommend taking it easy and let it heal. Once the knee is messed up it never really heals. I would recommend cycling, but poorly adjusted one can cause damage to the knees also.

UD

I didn't rest my knee soon enough when it hurt.  I thought I could run through it, until one day I couldn't.  I've done PT and I still can't run.  It's been almost a year.  Rest it now so you don't make something that may me innocent worse.  Try biking or using the elliptical, they are both easier on your joints.  

I have joint issues, largely from being overweight, not from repetitive stress.  RICE is the answer --

Rest

Ice

Compression

Elevation

Don't try to run until it doesn't hurt at all.  Focus on upper body strength if you want to keep excercising.

temporary relief can also be found with ice packs, but rest appears good advice too.

biking is great, but i'm beginning to suspect mine is not adjusted properly. My knee hurts, but i bike, do eliptical, walk and lift weights. i guess i'd better stop one or the other to find if that's the one that's causing my knee pain. it feels like a tendon on the inside of my right knee. thanks for your post.

make sure your doing strength training too if your running. It's made a world of difference in my knee pain. Be very careful and see a specialist if it doesn't heal up. I pushed through mine stupidly until i was getting off the treadmill and could barely walk. turns out i have a Baker's Cyst.