Suggest Treatment For Tightness Sensation In The Chest

hi, first I am a 35 year old female, I am very active and in very good shape. I have been running 3-6 miles daily for almost two years and doing other exercises as well. The past 2 weeks I have had some issues when running/jogging. At first I thought it was because I spent all winter running indoors on a treadmill and in the past two weeks moved outside to run. But, when I run I get a tight feeling in my chest and have trouble breathing, not real bad, but bad enough that I have to walk to get it under control then I can continue to run again. This starts about 10 minutes into my workout and then when I walk it takes about 3 minutes to get back where I can continue to run and this just keeps going in a cycle. Yesterday since it was too cold to run outside I ran on the treadmill inside and it happened there too, so my previous theory of moving outside I don t think is correct. I am not overweight 5 2 118 lbs and as stated before in very good shape. I just need to get an idea of what is going on. Thank you!