I'm a 20-year-old male, underweight (genetically, mostly) and out-of-shape, but relatively healthy otherwise. To make a long story short, in the past 3 months I have suffered from a number of symptoms due to anxiety. I saw my doctor, who performed a physical and a number of blood tests (fasting lipid, metabolic panel, thyroid, etc.) Everything came back normal. Then I ended up in the ER a few weeks later due to a panic attack, where they performed a resting EKG and chest x-rays. Those came back normal as well.
Seeing as I haven't had any sort of official exercise program since high school, I decided to push myself to get back in shape. This last week I started up a cardiac program: My goal is to powerwalk the neighborhood at least 3 days a week, for 30 minutes each day. Sunday was the first day, which went absolutely fine. Yesterday, however, was a bit different. A few minutes into the walk I decided to test myself to see what my heart rate would be like if I went for a brief sprint. I sprinted for like 15-20 seconds and checked my heart rate (about 165 bpm). I then continued walking. A minute or two later, I started to get a deep chest pain that ran from the center of my chest up to my collarbone and the base of my neck. It followed me the rest of the way home (another 25 minutes or so) to varying degrees, stopping almost immediately when I walked in the door.
I know it's probably irrational, given the tests I've had and my age and lack of risk factors, but I am concerned. Does this sound like I have some sort of heart problem, or am I just out of shape?