Suggest Treatment For Left Sided Headache
No reason to panic for now
Detailed Answer:
I read your question carefully and I understand your concern.
Headache is a very common complaint, affecting almost all of us at one time or the other during our lifetime. The causes can be so many, from common flu, stress and anxiety, ear or sinus infection to more threatening ones like stroke, tumor etc. So only from the complaint of headache it is difficult to judge on the origin. If no other symptoms, since it's been only 12 hours, over the counter painkillers like acetaminophen or ibuprofen and watching its evolution is the recommended initial approach.
Elements which would warrant more prompt attention would a very acute onset of headache (we call it thunderclap headache), very intense, increasing headache, visual disturbances, behavior changes, confusion, weakness of the limbs on one side (esp if on the right), fever.
I remain at your disposal for further questions.