No, just because you feel pressure in your sinuses, it doesn't mean you have an infection. It just means they are full of snot. Drink more liquids to thin out the mucos, if you like Tea, drink some Breathe Easy tea.
Also, try some Sudafed. But yeah, if the snot is green or something, that would make me think it was an infection. But, since you just feel presure, I would say they are just full of snot.