Hi. What made you get the ekg? What symptoms were you having? And where did you have it done? ER or cardiologist? Sinus tachycardia means your heart was beating at a faster than normal rhythm. 60 to 100 is normal. Different things can temporarily cause that including stress, fever, medications, anxiety, caffeine, etc. But if you have any other symptoms with it, you need to immediately investigate. Particularly if you have shortness of breath or chest pain! Is this what brought you to the point of the ekg? It's often prevented with lifestyle things like regular exercise (that doctor has approved), heart healthy diet, proper weight, etc.
That's the surface stuff. But there are different types of tachycardia. Do you have any more information on that or when is your doctor's appointment? Bottom line is that at times, it is normal to have an increased heart rate and it would be expected but prolonged periods that can't be traced to exertion, anxiety, etc. become something a doctor will want to dig into.
Here's a big long article on everything and more you'd ever want to know about sinus tachycardia. https://www.mayoclinic.org/diseases-conditions/tachycardia/symptoms-causes/syc-20355127 But it would be most helpful to have more information.
Atrial enlargement is often unnoticed but happens due to underlying conditions. https://www.healthline.com/health/left-atrial-enlargement
What kinds of symptoms have you been having? I think it is wise to work with a cardiologist to address what is going on or uncover if there is underlying heart disease.