What Causes Weakness And Shortness Of Breath While Exercising?

I am a 66 year old woman. I retired last summer and have been taking fitness classes three days a week that involve aerobic exercise, strength training, and flexibility. I think I should be feeling stronger now, however, I feel very weak and get short of breath while exercising. My family doctor did all kinds of blood work and everything was normal . I went to see a cardiologist and my heart checked out fine. I don t experience any chest pain, just have a lot of trouble catching my breath. I have to stop exercising until I do.