You did the right thing by seeing your doctor. You're doctor's probably right that you don't have a heart problem given your age and the fact that your tests came back normal. Some things that can increase your heart rate are anxiety, exercise, fever, caffeine, even consuming certain types and amounts of food. As for taking a hot shower and feeling your heart rate increase, it's your body's way of responding to the heat. You should try taking a cooler shower. I have the same problem because I'm very sensitive to the heat.
Try not to worry about it because that will cause more stress which may in turn cause more rapid heartbeats. Good luck