The knee is a very complicated joint, and what you are describing may not be fluid on the knee.
Every joint in the body is surrounded by a fascia containing synovial fluid- this helps lubricate the joint articulation, much like oil in an engine does. If a joint becomes injured, the bodies response is to send healing white blood cells (lymph) to the area- this stuff a)begins ther healing process and b)causes swelling and stiffness in an attempt to stop you using the body part- a bit like a natural brace or splint. Trouble is that the body always overdoses you with this stuff and you need to mobilise and ice the area to minimise the sweling so it doesnt end up hampering your recovery.
The 'popping' noise you hear o deep squats could cause inflamation of the synovial membrane, or it could be putting too much stress on your quad tendon, or it could be an indication of wear and tear on your meniscal cartiladge, or it could be an indicator of chondromylacia patellae- knee cap being out of line due to imbalance of strength between medial and lateral quads (also known as runners knee).
Thing is, prior to medical diagnosis you need to utilise common sense.
1. Do not do movements that you know aggrivate the problem. If this means adapting some stretches, or not going as deep in stance as usual, explain why to your sensei, and I am sure he will understand.
This goes for personal exercise as well, for now only do partial squats, stopping long before pressure on the knee is a problem. No running for now, and be careful of twisting the knees when throwing punches at your wavemeaster, make sure you pivot on the ball of the foot (I know that is proper tech. anyway, but its easy to forget)
2. Warm up extra thoroughly, including good stretches of both quads and hamstrings. After exercise ice and rest the knee even if you dont think you aggrivated it. No harm in using tubigrip or a knee brace in the short term for extra safety either.
3.A general practitioner will prescribe painkillers and rest, but this is not enough. Request physio consultation, exaggerate a touch on the severity of the pain if you must, but get this properly diagnosed and a firm specific rehab plan in place to prevent it becoming chronic.
There you go, now hop to it (pardon the pun)