Sounds like your form is off, and you've got problems that are
aggrevated from the overuse of the knee.

I'm currently undergoing physical therapy for something similar.

Best way to take action on something like this.

1.  cease the exercise and excess strain on the knee for a few weeks.
2.  take anti-inflamatory meds to help with the swelling
3.  Ice the knee to help reduce the swelling.
4.  If pain and swelling do not reduce in a few days, call your
doctor.  If the pain continues for more than a couple weeks without
exercise, call your doctor.

You could have done anything from torn your ACL to simply having
Patella tracking disorder.

http://www.stumptuous.com/pf.html

I'm no doctor, but I would highly suggest you see a doctor.   You are
over doing it with the exercise for whatever reason.   Getting the
problem diagnosed means getting proper treatment, then you can work on
form and exercises to strengthen the outer muscles and work towards
repairing the knees so you can do the exercises.

yes. I went to an orthopedic surgeon for a very similar pain issue.

xrays were negative, but he didn't do an MRI so I don't know what's
going on with the knee.

You may have just torn cartilage. Mine healed up well enough, but
there's still little twinges of discomfort sometimes.

Lots of things can blow out in the knee area... the meniscus, the
flimmery-bobs, etc. Surgery is required to fix the more serious
issues, and arthrocopic surgery can clean out ragged edges that are
causing discomfort.

Also, I just started a Glucosamine / Chondriotin pill regime today. 3
pills/day for 3 months, $30. Not sure if it will help, but it
shouldn't hurt at least.

It might be a muscle strain or tear.  Ice it, and when you feel better
gradually resume - don't exercise if it hurts.  I take glucosamine -
which really stopped my knee joint pain. Elliptical trainers are the
least knee strain for me.  General rule - don't increase your workout
by more than 10% in any given week - to avoid injury.

You know, this sounds like my *new* problem.  I've done the stationary bike
two times this week to take the pressure off my totally trashed right knee
and starting last night I couldn't straighten my  left knee without
pain...couldn't even walk with a straight leg.  Had to hobble around with a
bent leg.  This I didn't need after a doctor's visit in which I was told to
not compete next week and to not lift anymore until I get the right knee
fixed and rehabbed.  Sigh...Hopefully JayJay is right and the pain is just
from overuse.  I plan to ice it.  You know, I was planning to go to
Nationals even if I couldn't lift because everything is paid for....but now,
I couldn't even walk through the airport!

If I had any advice for anyone reading this group, it would be to get the
weight off immediately, so you don't end up with lousy joints like me later
on in your life!!

Elise