Bill Walraven
Most probably you have an allergy, where the rain lays down the dust(pollen). Sinusitis you can spot by thumping with the ball of your hand against the knob you feel at the back of your head.This causes a kind of backlash to the mucus in your sinuses and propels it momentarily against the nerve it has been bothering inside your sinus.If you feel a sharp sting,bingo its sinus.If not, it could still be.But I guess it's pollen.