THC is what increases heart rate, likely because it lowers blood pressure so your heart compensates by beating faster. Smoking has little to do with it. Vaping can actually increase it more because you get more THC and less CBD. CBD can help keep your heartrate low because it counters some of THC stimulating effects. That's why indicas are recommended.

Also, weed lowers blood sugar, and low blood sugar levels can increase heart rate. So eat a meal before you smoke and drink orange juice if you think your blood sugar is low.

I once vaped and my heart rate went up to almost 200. I was also really scared and my blood sugar was definitely low because my skin was pale, was trembling but not cold, and was disorientated (search low blood sugar effects). 120 is no big deal and very normal for weed.

Smoke less, don't smoke sativa dominant strains, eat and drink to keep your blood sugar up, relax.