Have you noticed that since working out your resting heart rate is lower than it was? This is no cause for alarm as most athletes (yup, you're an athlete now) generally have low heart rates.
...consider a bricklayer lifting bricks. If his arm muscles are strong from lifting lots of bricks, he can move 10 bricks with each lift rather than just two or three. Similarly, if your heart muscle is strong thanks to running, it has a higher stroke volume, which means that it can pump more blood with each beat than an untrained heart. It can also pump the same amount of blood in a minute using fewer beats.
The average resting heart rate of adults is 60-100 beats per minute (bpm) while athletes are around 50-60 bpm. Other numbers to know: The infant/neonatal rate of heartbeat is 130-150 bpm, the toddler's is 100–130 bpm, the older child's is 90–110 bpm and the adolescent's is 80–100 bpm.
The worry would (and should) come in when a resting heart rate is lower than 30. However, even that can be completely normal (and usually is) if the athlete is totally healthy. The slow heart rate usually just indicates a strong heart. If you are concerned, it never hurts to consult a physician when dealing with your health.
Fit's Tip: The best time to determine your resting heart rate is when you first awaken in the morning or after sitting down and relaxing for at least 5 minutes. Get a heart rate monitor to monitor your heart while working out. Don't know your target heart rate? Check out the Fit Calculator to find out!