What Causes Persistent Headaches?

hi, iv been having severe headaches for the past 3 weeks. the navy doctor on my ship said i had an inner ear infaction and possibly a sinus infection even though he didnt check. they gave me amoxicillin which i took the last of 5 days ago, and im still getting severe headaches lasting from 5 hours to 2 days or more. iv gone back to medical and they said theres nothing else to do for me. what should i do?