Hi,
How are you? Can you elaborate on your headache? Was there a previous trauma to your head? I agree with Dame that additional information will help us determine the possible cause. If rest and pain medications do not help. you may benefit from further evaluation by your doctor. Your complete medical history and direct clinical examiantion are important. Take care and regards.
You're not giving us much information to work with. Where on the left side is it painful? What kind of pain is it? (dull, throbbing, sharp, etc.)? It could be something simple like a tension headache or it could be something more severe. However, with almost no details, we can't help you.
~Dame