That's a real common response, 4 2 0, especially if you're smoking sativa-heavy weed. Smoking of any kind, whether it's cigs or weed, causes your heart to have to beat faster anyway because your heart has to work harder to circulate oxygenated blood and counteract the effects of inhaled smoke. But sativas and sativa hybrids, which are the ones that are most commonly sold on the street, also cause an "adrenergic" response in many folks because they have a slight stimulant effect. The adrenergic period is where you get a little extra burst of adrenalin and your blood vessels contract, your heart pumps faster, and your blood pressure goes up. This is the phase during which a lot of people feel panicky and anxious. Sometimes this phase can cause trouble for people who have pre-existing heart disease or rhythm problems, but for most folks it's harmless and it passes fairly quickly. As you get more accustomed to smoking, it generally becomes less of a problem. If it continues, try switching strains to something more soothing.