Mefenamic acid is a NSAID while Amoxicillin is an antibiotic. Though there is no drug interaction but it is better to get the root cause behind the pain diagnosed and then start with the medications. So, i would advise you to visit  a dentist, get an Xray done and then start prescribed medications treatment. Till that time you can try some home remedies like placing garlic and salt paste over the area or placing ginger....to know more of such remedies visit the link http://www.identalhub.com/articlecategory-home-remedies-for-dental-pain-41.aspx