The fact that the symptoms have only been going on for a week is encouraging. However, rarely, severe headache
can be a sign of a serious problem, such as a tumor in the brain. Therefore if the headache persists, if you have vision changes, nausea and vomiting, or trouble with balance and coordination then you should see your doctor
immediately for evaluation.
Aside from serious causes, which are unlikely, one possible cause here would be sinus congestion. If you have nasal stuffiness, runny nose, cough, or cold symptoms this is by far the most likely possibility. Sinus congestion occurs when mucus plugs the sinus allowing buildup of fluid and pressure. It usually responds to decongestants and nasal saline irrigation.
Another possibility would be migraine, especially if you have a history of headaches going back over your life. Classic migraine does occur primarily on one side of the head and is typically a pulsing pain (as opposed to sinus pain, which tends to be more steady and aching).
Start by making an appointment to see your primary care doctor
. They will perform a full physical examination and determine whether or not you have any symptoms that require further workup or treatment.