Can an impacted upper wisdom tooth cause mild headache (slight pressure) for 1 month on one side of the head on the forehead, above the eyes and a bit on the neck? if not, what else could cause that?

Yes, an upper wisdom tooth can cause those symptoms.

Yes. Also sinus infection and occipital neuralgia (https://www.aans.org/Patients/Neurosurgical-Conditions-and-Treatments/Occipital-Neuralgia). Muscle spasms of the neck are a common trigger. Consider removing the tooth. If the headache persists, try neck massage. If it is still there after that, see a neurologist or your family doctor.

Add to Dr.Pelton's list of possible causes infected maxillary teeth, advanced inflammatory gum disease, history of trauma, pernicious habits such as bruxism, malocclusion, and allergies. See your Dentist for clearance. If no pro-facial causes are found, consult with Neurologist.