Knee pain during bike - any help?

Hi everyone,

First season as a triathlete, and for that matter it's my first season as a biker, runner and swimmer.  I've had a few aches and pains along the way but it's been a great experience...however, in the past month I've run into some knee pain that can be absolutely debilitating at times and while I'm planning on visiting the ortho at some point in the next week I wanted to get some feedback from the community, anyone who's dealt with something similar.

It first begin a little more than a month ago, during the B2B ride from Boston, MA to Windsor, VT.  Because of a ton of early summer rain and a hectic schedule--both at work and out--I didn't quite get the training in that I probably needed to feel comfortable on the whole ride.  At about mile 95 I started to feel a dull pain in the middle/lower portion of my knee.  It slowly, slowly started to increase in intensity--not to the point where I couldn't continue but it certainly hurt.  When I got off the bike I noticed it hurt more walking, and stairs gave me problems.  I had it checked out by the medical staff at the end of the 150 mile ride and was told there was nothing structurally wrong, and to just pound down the Advil and ice it. 

I did that for the next few days, took a full week off the bike (didn't run, either).  It slowly dissipated and the past few weeks it didn't hurt at all.  I did a slow, slow 70 mile bike a week ago without any problems, but then on a rideof 120 miles to the Cape it started back up--dull pain on the bike, same spot on the right knee, followed by alot of pain once off the bike.  It did, however, disappear very quickly--within two days--so I went for a 50 miler today.  I was fine until the last half mile, when it literally just exploded into intense pain, I basically did a slow spin into the finish with my left leg, and immediately threw down a few Advil.  It actually isn't bad right now (4 hours later), going up and down stairs theres a slight pain and if I move it after it being immobile there's a slight pain but nothing bad at all. 

My question: has anyone dealt with this in the past?  I talked to the guys I rode with to the Cape and they mentioned keeping a higher cadence, but I really don't think that's it: I generally sit at 85-95, and watched people closely today and I was definitely at a cadence as high as anyone else out there.  I'm guessing it's tendonitis but am wondering if there's something else that it could be.  I'm still pretty young--29 years old--and haven't had any knee issues in the past so it's rather discouraging finishing up the Cape ride with guys almost double my age and being the only one needing ice at the end.  Could it be as simple as me not being prepared for the distance I rode to VT, and a similar unpreparedness on the Cape ride?  That'd be best case scenario, of course, as I would just have to know to fully work up miles, but I'm worried it might be a recurring thing that I'll just have to treat very carefully.

I'd love to hear if anyone has any suggestions or thoughts on it, I really can't be taking days off--let alone weeks--as I have an important sprint tri in 4 weeks followed a week later by my first OLY so I just have to get miles in.  Thanks!