THC is a vasodilator (it makes your blood vessels relax and become larger) which is thought to cause reflex tachycardia which means your heart has to beat faster in order to keep blood circulating because the "pipes" (blood vessels) have suddenly become larger. The vasodilation is also the cause of bloodshot eyes.

There is also something to said for the anxiety which high THC cannabis can cause, as well as a sort of circular anxiety due to the fast heart rate (my heart is beating fast due to reflex tachycardia, which makes me anxious and worried, which makes my heart beat even faster, etc.).