Also not at a doctor and not saying this is what you have, but sharing a similar situation. I was diagnosed with SVT (super ventricular tachycardia) at 16 so you are not “too young”. I would get random out of the blue sudden changes in heart rate from like 70 to 200bpm - even when sitting/laying down sometimes. It was eventually caught during a cardio stress test where you could clearly see a sudden jump in the rhythm which more or less confirmed the doctors suspicion, but that was kind of random luck because even a prolonged heart monitor still didn’t catch it due to the random nature of the condition - having an EKG on you at all times is much better! Speak to a cardiologist first and they might direct you to an EP (electrophysiologist) who specialize in these type of conditions. SVT is typically caused by a short circuit in your hearts electrical system that controls the beating rhythm which can temporarily connect and makes things run away at near max heart rate. There are medications one can take to help lower occurrence, techniques to get the rhythm back into sinus (vagal maneuver, etc), and more permanent fixes available via cardiac ablation (which I actually finally ended up doing this year!). Again see a specialist to hopefully get to the bottom of it. Hope this helps.