I always had a great cardio. but nowadays i tend to get tired very quickly in jiujitsu. could it be asthma?how do i know?

Fatigue: If you are in good shape, the fatigue should be unusual. In this situation asthma (or exercise-induced bronchospasm) might very well be the cause. This is usually perceived as shortness of breath, not being able to take a deep breath, or as wheezing (whistling sounds in your chest). A doctor can do breathing tests (peak flow) to assess or you can try a bronchodilator (inhaler) like albuterol.

Wheezing?: The hallmark symptom of asthma is wheezing--you may feel tightness in your chest, and hear a whistling sound in your chest. It may be hard to inhale and even harder to exhale. Your family might even be able to hear you wheeze. Cold air could make it worse. There are other things that can cause fatigue and shortness of breath while exercising. See your doctor if it continues.