Jaw pain only when i open my mouth.

For the past 3 days, I have had an increasing pain in the right-side jaw area, primarily lower side (starting from the under-chin and reaching to the ear-lobe). The strange thing is, that there is absolutely no pain when my mouth is shut - even when i press the area. However, when i open the mouth, the pain appears and grows as i open my mouth wider.
No temperature/fever, and no tooth ache.
Can this be a dental problem (i have an un-filled upper moler on the same side, but it has not given me any problems so far). I would have assumed that if it is dental, it should cause pain regardless of whether i open my mouth.
Or is this more likely a neural or muscle problem?

Any input is welcome as it is a weekend and I must choose between waiting until Monday or seeking emergency help.

Regards
Christo

[Ramin Mehregan, DMD]

Dear Christo
Based on what you describing it could be both muscle or tooth. If the pain is getting worse and you experience swelling, I would recommend you seek emergency help, if not see a dentist on Monday.