Hey. Whenever I Check My Pulse (heart Rate), I Notice

Hey. Whenever I check my pulse (heart rate), I notice that it'll beat at a normal rate and then speed up for about 2-3 seconds, then go back down. This basically happens anytime I check my heart rate. I am 18 years old, male, and in good physical condition. Although, I have been underweight my whole life (in my opinion), and I have always been skinny with fairly small muscles. Do you have an idea of what might be causing this and if it is dangerous or not? Thanks.