Sinus tachycardia = Normal heart rhythm, just rapid (above 100 bpm).
The nonspecific ST abnormality can have a variety of causes, but often with a rapid heart rate, you may get EKG changes. It can also happen with anxiety and hyperventilation.
If you are nervous, ask for a stress test. And if your doctor finds your EKG to be normal, try to accept that.
i do also have a seasonal midsternal pain that reaches my left armpit line, i did not notice it not until last year that thru excessive laughter it aches so much that i bend down to my knees and cry, and now it hits when it strikes, it causes my left shoulder pain also and sometimes the pain goes thru my left back that surrounds the lungs