I have pains in my chest whenever I climb stairs or do strenous exercise. What is wrong with me?

I'm not a doctor and can't prescribe treatment, but if it were me I would see a doctor about it. In the meantime, stay away from strenous exercise. Has your weight shot up recently? If so you might have swelling. Check your heart rate. When doing exercise, your HR shouldn't be higher than 70-75% of your max, where your max is approximately 220-your age. If your HR is higher than that when you exercise, head for a doctor asap. If your HR is higher than that when you're sitting quietly, head for a doctor faster than asap (use the ER of your hospital).