Chest Tightness, Pain On Breathing Post Working Out. Treatment?

I m 19 years old and I had a bad workout about 3 weeks ago, and since then my chest hasn t been right. It feels tight and there is always a dull pain. Sometimes this pain increases, but it never spreads. Always in my chest, near the middle, a little to the left. Any sort of physical activity (such as carrying groceries through town) can make it hurt terribly, and it doesn t get back to normal until a few days after. Sometimes breathing hurts. It hurts most when I take deep breaths, but it s usually not enough alone to trigger an episode. I ve gone to a hospital, and they confirmed it was not a heart attack . However they haven t confirmed what it is at all. They say everything has come back negative. I m scheduled for more testing and maybe 48 surveillance but the pain is very real, and I don t want to be written up as a Hypochondriac. I m looking for a fresh perspective from an expert.