It sounds like you're developing an anxiety problem. You have the symptoms. It's unlikely you have a heart problem at your age, though not impossible, but even thinking you do just because you've gotten obsessed with checking your pulse -- you made it this far just fine without doing that, right? -- shows something is bugging you and making you anxious. It might be time to see a therapist who specializes in anxiety treatment to nip this kind of thinking in the bud.