I have had a nonstop headache mostly on the right side for about 3 weeks? doctors say its sinus but could it be a brain tumor? its relieved with advil (ibuprofen)

Probably not: But certainly if it continues...It needs further workup.

Unlikely: Headaches can come from fatigue, sleeping too much or too little, eye strain, sleep apnea, snoring, being dehydrated, having chronic sinusitis or allergies, too much caffeine or over the counter headache medication, high blood pressure or something more serious like a tumor. See your doctor and get checked if it doesn't get better in the next month.

See a specialist: It is hard to know by your history and description. What you have may come back. I would see doctor to rule out other issues.

Highly unlikely.: Not very likely, but certainly possible. Brain tumors can cause headache's, but so can dozens of other things. If normal treatments for sinusitis are not working, there may be a reason.

Not a brain tumor: Primary headaches can be very debilitating but doesn't mean there is a tumor in most cases. If advil (ibuprofen) helps its most likely a tension type headache. You can try some self care things including warm compresses, hot showers, meditation, massages and green tea.