I'm 17 years old. I was once sitting on the couch and my heart felt like it stopped a second did a double beat, then a big beat and went back to normal. It took my breathe away. I never thought much of it, but it's happened more frequently now. I feel lightheaded afterwards usually. The other thing is in the past year I get this reoccurring tight chest feeling where I feel like I'm not getting enough air. When I take large breathes through my mouth it feels wonderful but the feeling normally remains after.