My daughter is suffering constantly of head pain behind her head and on the left side. .please help could that be something serious?

It is serious if it : harmfully impacts her daily function in academics, peer interaction or behavior. Keep a headache log for her pediatrician with onset, frequency, duration, severity, environmental circumstance & response to Ibuprofen. Does it wake her up? Any dental or ENT symptoms? Physical & neurological exams determine need for MRI of the Brain, referral to a pediatric neurologist, or other course of action.