I wouldn't worry alot about the term Sinus Tachycardia anyone including me with a transplanted heart has that. It means you have a normal sinus rhythm and tachycardia means fast heart rate. Mine averages 100-110 resting and can normally go up to 140 when im like climbing the stairs. Before my transplant tho i had ventricular tachycardia which is very dangerous. My daughter went through a similar case as yours her heart rate at rest with a normal heart is around 100. She went to my dr that also treated me back when i had a bad heart and he couldn't find anything either. He feels she has a lot of anxiety and needs to find ways to relax. So a second opinion is an option but maybe its something not heart related. For a healthy young person like yourself your heart while exercising can easily go ip to 70-80% of your max and at 21 your max is 220-21 or 199. Now you would not want it to stay there very long but you got some room. The say while exercising you should shoot for 70-80% of you max. My only concern is when you say it was 170 while resting and it was a normal sinus rhythm. Very odd. One thing for me i know is when I'm a little dehydrated my heart rate avg goes up. Do you drink plenty of water? That would also help lower your heart rate. Let us know what you find out?