Test it by slowing way down on the weed and see. It's easy enough to check that out.

Sounds like either you're responding with the "adrenergic" reaction, which is very common, especially right after smoking, to the particular strain you're using right now, which I'm guessing is a sativa. That's just the heart-racing, dry-mouth, slighty flighty/anxious feeling that happens when the smoke first causes some vasoconstriction and makes your heart beat faster and gives you the same feeling as if you've had a burst of adrenaline. Or else you're having some anxiety along with your high. Both reactions are very very common. And just because they haven't happened before doesn't mean they can't start anew.

If you cut down on weed or take a two-day break and notice that the symptoms ease, you'll have your answer.