Severe Headache Behind Eye

I do not suffer from constant headaches, but when I do get them lately they have been on one side of my head. For the past couple of days my head has been somewhat bothering me. I honestly think a lot of it has been my sinuses. One of the days I was somewhat dizzy and today I have felt drowsy and had some pressure in the middle of my head. The pressure went away after I took a shower, which I have heard can help if you have sinus pressure. However, tonight I got a headache on the right side of my head right behind the eye. It was quite severe; however, not bad enough to make me lie down or anything. Still, I've gotten these before, this being the worst. What causes these types of headaches and are they anything to be worried about?