Left side chest pain for months? What could it be?

Well, it's not a cardiac problem or a lung problem because they would've been picked up by the x-ray and the cholesterol test. If you play a lot of sports, you could have damaged the left pectoral muscle (pulled it). I would simply advise you to relax, maybe stay off of the sport for a week and if it's still bothering you, go back to your doctor.