You have slipped or mis-aligned by a tooth. The problem might be in the distributor timing or the cam timing, likely the distributor. Next time you install the belt (soon, probably), get the cam set (can't miss that mark), the crank set (harder to find the mark but it is a big raised vertical ridge on the rear cover up against the block) and then make SURE the rotor points directly at the tic mark on the lip of the distributor. Then and only then, button things up. No one but Mike W (pointed it out to me) and I use it but it is fail safe. That is why we have 2 or 3 questions a week about timing belts maybe done wrong. It gets everyone now and then.