Since you mention of a strained jaw present since few weeks it could be a muscle spasm or a strained joint. Since it's difficult to analyse a cause here, no definitive treatment options can be given.
Next Steps
I strongly suggest you seek an appointment with a dentist, will help reduce or aggravate further problems.
Health Tips
Home remedy would be medication to relieve pain(common pain relief medication for headache). If there is no evidence of any swelling, moist heat with either a hot water bottle or a warm moist cloth will help reduce pain.