What Causes Sharp Pain In The Upper Chest Area?

Months ago I noticed sharp pain in my upper left chest that went through to my back -- especially when doing a HIT class. I figured I pulled a muscle and carried on. I began noticing I would get out of breath quicker and it was harder to recover. I chalked this up to Bleomycin lung. However, over the past few weeks, the pain is almost constant and I have a feeling of pressure on my chest. I have been out of the gym for about 10 days (taking care of kids with strep and trying to see if this got better), but it continues to get worse. Now, after a short walk to the car or a flight of stairs, I feel extreme pressure and difficulty getting a good breath. What could it be? (I m a 40 y/o female -- non smoker, non drinker)