It sounds like you have strained the patella tendon, otherwise
know as" Jumpers Knee" . The reason its the left knee is this is
the leg you land on continuously during a match when you serve.
Here are some recommendations:
1) Before and after you play stretch your quad by placing your
left foot on top of the net-this will also stretch out you patella
tendon. By not stretching your quads this will eventually cause
pain in your patella tendon-stretching is critical.
2) Start taking anti-inflammatory medicine such as advil, and make
sure you ice your knee after playing-this is very important. Also,
consider using a heated whirlpool and heating pad throughout the
week and before you play and stretch the knee.
3) Strengthen your legs around the knee by lifting weights-squats,
4) Do a search on google under patella tendinitis or jumpers knee-
you will get some great information from rehab. centers.
5) I f the pain doesn't go away after 2-3 weeks after doing the
above go to a physical therapist because you will get access to
special treatments such as ultrasound that will speed recovery.
My guess is that if it is not a serious acute pain the above advice
will clear up the pain, especially the strecthing and strenthening.
If it is not to painful you should still be able to play-if the pain
doesn't go away take some time off . Good luck.