Im 16 and female and weigh 140 pounds. I have shortness of breath while sitting. When I stand fully relaxed theres an extremely heavy weight on my chest and it requires a lot more effort to breathe and if i?m not trying to breathe I only take shallow breaths. It?s extremely uncomfortable but not painful. When I lie down this feeling completely goes away. It?s only there halfway when i?m sitting and I feel the full force of it while standing. I feel like my muscles twitching like a heart palpation but without pain and multiple areas on my upper chest. On the left side and right side. Sometimes I can even see and feel it through my chest while touching it with my hand. I?m taking no medications except vitamin c gummies. My nutrition is poor and I don?t have a lot of muscle. My resting heart rate was around 40-50 a few months ago but now its at around 60. I was also diagnosed with sinus bradycardia when I measured my heart rate. I also had a heart murmur when I was born but it was treated and fine after.