What Does The Left Enlarged Atrium Mean?

have been having episodes of hypertension and cardiac pain. I spent Tuesday night in the ER getting blood work done, including a hs-CRP test which came back elevated at 7.4 mg/dL. I had a troponin test done that came back as slightly elevated (0.9 ng/mL). I also had an EKG which showed tachycardia (resting heart rate up into the 160s) as well as episodes of a-fib, and an echocardiogram which showed my left atrium was significantly thickened/larger than expected for a normal twenty year old. I was admitted to the cardiac unit over night for observation and given nitroglycerin for the pain as well as medication for my blood pressure. The blood pressure levels regulated somewhat, going down to average of 138/95 at discharge. They gave me a preliminary diagnosis of angina which is shocking since I m 20, of normal weight, have a healthy diet, and normal cholesterol. I ve been referred to a cardiologist and am scheduled for a stress test on Monday. What should I expect? What will the stress test show? Also, what does a enlarged left atrium mean? Is it serious? thanks!