You should ice it and see your doctor. You can take Tylenol for the pain. Follow directions.
Put ice on it & go to the Doctor.
If your knee is swollen, it is almost certainly painful too. This often results in you trying to protect that leg and you start moving differently to normal which puts more strain on your good knee, which may then develop pain. The best way to reduce swelling is to elevate, compress and ice your knee - see the related link below for more information on how to do it safely
This can sometimes be caused by a blood clot in which case you should get medical help as soon as possible.
Painful.
PAINFUL
Torn legaments
Take it to the vet.
There are several websites where an individual can find out how to tend for a swollen knee. Examples would include WikiHow, WebMD, Native Remedies, and Livestrong.
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The appearance of painful and swollen lymph nodes is another reason for consulting a doctor.
A swollen knee should be treated by putting ice on it three times a day for twenty minutes at a time, soaking it in a warm bath and wrapping it in a compression bandage.
Cellulitis
For swollen joints you need ice and maybe a lukewarm soak in a bath of epsom salt.