Im A Marathon Runner. I Noticed About 3 Weeks Ago

im a marathon runner. I noticed about 3 weeks ago when I was training I was not able to take in a deep breath and therefore my legs and body felt heavy with a lack of oxygen i have also been getting slight chest pains. The doctors have done blood tests, x rays all come back clear. I find it hard just breathing even at work.Im unsure of what to do.