Hi Justxlouise and welcome to our group,
What you described sounds like exercise induced asthma. Often the easiest solution is to take one or two puffs from the inhaler (albuterol) before exercise. This will often relieve the symptoms that are brought on by the exercise.
Asthma can disappear and return later in life (it happened to me) and it also can happen that normally you won't show any symptoms until you are active. Talk to your doctor about using an inhaler before exercise. If this doesn't help, it may be that because you have reached your body's limits that are less than average because of thal minor. Taking folic acid supplements may help some but you may have to learn to stay within your limits. Tennis star, Pete Sampras, is also a thal minor and after retirement he admitted that his endurance was limited by thal minor, so his tactics were to finish off his opponents as quickly as possible in order to conserve his energy. Testing your hematocrit or hemoglobin level can help you know how much thal minor is affecting you.