Knees are pretty complex things and no matter what anyone on here says, they're not going to be able to diagnose anything over the internet. Knee pain can happen for host of different reasons. It'd be best to see a doctor or a specialist (maybe a physiotherapist or orthopedic doctor? - best to start with your regular doctor and he can refer you to someone if he feels he needs to).

If you think it's swollen or anything, ice it, try to keep it elevated when you can, and take some ibuprofen.

I hope it gets better soon!

Go reap the benefits of "free" healthcare and have them checked out by the doc. If it's nothing externally apparent, then you will probably need an x-ray or other scan to figure it out.

I have that problem too sometimes.
If it has something to do with your bones, I say eat a calcium supplement or something.

That or go see a doctor.

Check your diet, you may not be giving your body the proper nutrientes / minerals etc.
Check with a doctor, get it scanned.
You may have a broken bone and not even know it.

You should go see an orthopedic doctor.
If for some reason you can't right now, you could get knee braces. They're sold everywhere.
They really help.