First things first. Practice a little RICE(Rest, Ice, Compression, Elevation). Give it a several days of not using it hard. Next back off of any high impact (i.e.running) or heavier than body weight lifting. If it starts to feel better you can slowly start increasing your workout again. If it doesn't, continue to limiting lower body exercises, or go see a doctor/treat it easily for 2-3 weeks ( I normally wait 2-3 three weeks before I consider seeing a doctor, unless the pain or instability is very noticable).
If it was a ligament, chances are there wouldn't necessarily feel pain, but instead a sense of instability. If your knee has swelling(ie. non-serious strains/bursa sac, a torn miniscus, or cartilage damage the pain could be an indication of the injury sustained.
Do remember that a GP(general physican) doesn't specialize in that type of injury and may need to refer you to an orthopedic dr. And that if you have doubts a doctor is always better advice then us Internet nerds.