The posts are quite right, your heartbeat slows down in meditation or trance. There are two possibilities.
First, if you are about to project for the first time, you may be experiencing some anxiety because you are excited, and unsure of the experience.
Second, you could simply be experiencing anxiety about your heartbeat slowing down, - worried that something is wrong. Either way, it is likely to be an anxiety issue. Perhaps you could help yourself by having some chamomile tea before you next pratice projecting. Good luck :)