Trismus (Lock Jaw) caused by wisdom tooth - how serious is it?

It sounds like you've figured out that the tooth itself is OK and the problem is in the jaw... and you're right if the condition gets worse you should see a dentist immedialtely. With that being said, I don't think you have anything to worry about until your appointment. It sounds like your tooth erupting has caused you to have a mild case of TMD. You can ease the pain by taking an over the counter anti-inflammitory (like ibuprofin) and applying a warm, moist cloth to your jaw to kind of 'loosen' the muscles. The pain is most likely coming from the pressure being put on the joint in the jaw (TMJ) from the tooth entering the mouth. It may eventually subside but you'll probably see the dentist before it does. Good luck, and I hope this helped!