Weed is known to cause Paranoia in some people, which would increase your heart rate. it is not dangerous unless you have a pre-existing heart condition. It happens to me too sometimes, but am able to control it simply by telling myself "it's just the weed, nothing to worry about". I have actually heard of people quitting because if it, but again, it is generally not a dangerous condition. Just a side effect of weed for a few.