For Days I Have Had A Weird Pressure In The

For days I have had a weird pressure in the left side of my head, behind my eye. It happens a few times per hour. It doesn't hurt. It feels like a small balloon that grows larger and larger and then it goes away. At first it would go away completely but now there is lingering pressure all the time. It's not very bothersome. It just concerns me. What could it be?