Weed has an "adrenergic" effect right after it's smoked. Meaning you get a sort of adrenalin burst, which constricts your blood vessels and raises your heart rate and blood pressure temporarily. That adrenergic response is what causes the rise in your heart rate and your paranoia. The paranoia and the tachycardia (increased heart rate) usually go together. One doesn't necessarily cause the other. It's a temporary phase, luckily. Sativa-heavy strains cause that response more than Indicas.