I don't know where to put this, so if a mod feels the need to move it, go ahead.

One night my heart rate increased and it worried me. Then, I got shakey, a bit lightheaded, short of breath a bit and some chest discomfort. I went to the ER, and they ran some tests. Everything came back normal. Although the nurse said there seemed to be "elevations" with my thyroid and something else, but the doctor didn't mention that and neither did the papers the hospital gave me so I don't know whether or not that is important. I was told to go see my doctor, so I did. He told me the signs seemed to point towards anxiety attacks, since everything else had come up clean at the hospital. He gave me some Xanex and although they do work after I take them, I can't help but wonder if this is really anxiety related. I still get one of the symptoms from time to time usually being rapid heart rate and chest discomfort. I'm 20 years, 6'6, and weigh around 330-350lbs. I'm gonna go on a diet soon and get back in shape though, because this was a kind of wake up call that told me I can't keep letting myself go like this. Can someone help me out here? Is it related to my poor fitness? Is it anxiety related? Thyroid? Or something else altogether?