Increased heart rate is a common side effect. Unless you have some pre-existing heart problem, you shouldn't have anything to worry about.

Another side effect of the cannabis high is becoming more aware of your body and it's functions. Your heart is beating faster and you notice it more than you would normally. If you begin to get paranoid about it, your heart rate will increase even more, in turn feeding your paranoia, etc.

It's something you get used to and stop noticing as long as you don't let your imagination run away with you.