What Causes Fatigue And Breathlessness After Workouts?

Hi, I am a 35 years old female, 162 cm tall and weigh 68 kg, with no history of any health problems, I'v been practicing karate regularly for the past 2 years. The problem is that I get tired very quickly, in the begining I thought it was because Im new to this but til now I experience the same! after 5 min of doing fast moves I begin breathing heavly and is forced to stop to catch my breath. I tried many things that I thought might be the cause: resting before going to practice, not eating less than 2hr before practice, having small meals, trying to regulate my breath, but it is not working. I checked my lungs and did a blood test and all was ok. I hope you can advise me what to do. Thanks!