Your cannabis could have been spiked with something or that just the way it is. Some strains are pretty "racey" and have tendencies to get your heart beating faster, feelings of anxiety, or flat out pretty trippy. Each person is affected differently. When I first started smoking any of the Skunk #1 or skunk hybrid strains would get my heart going. I found it best to just relax and think calm. It's just a plant, don't worry about it so much. On the other hand, my girlfriend never has felt like her heart was beating faster when smoking these same strains. Regular smokers are usually able overcome these strains or avoid them to maintain an enjoyable experience.