You could have Asthma. My brother, (excellent in shape) swam a lot, he was in excellent condition, after he would swim laps after laps. He would get a shortness of breath. He later found out, he had asthma. You could very well have asthma.

There are many possible reasons. As the first responder noted, you may have a medical condition. Most doctors and trainers recommend that you see a physician who can evaluate whether exercising is OK, especially if you are older and/or have little or no history of exercising. In general, though, some shortness of breath is normal, depending on how hard you've exercised. As you physically work harder, your body needs more oxygen. Oxygen is supplied to the blood stream by the lungs, so with a heavier work load and oxygen demand, the lungs work harder. With exercise, take it slow at first. As you become more fit, the oxygen supply mechanism becomes more efficient.

The best advice somebody can give you is to consult your doubts with a medical doctor. Shortness of breath after exercising can be caused by dozens of reasons, from a bad present physical state (due to lack of exercise in a long time), passing thru too many cigarettes and ending on asthma or a very bad pulmonary condition. So don't play with your health. Ask a Doctor, and he will start asking the basics (age, clinical history, physical history, family background, etc) and giving you professional advice. We all hope this is only that you are pushing your body too far :)

While these answers are all correct, you should be aware that shortness of breath after exercise is also quite normal. When someone descibes being fit or unfit, this is actually a reference to how quickly breathing returns to normal... the quicker your breathing returns to normal, the fitter you are. As to why one pants in the first place, this is because your muscles need oxygen to work. In vigorous exercise, the muscles need more oxygen, the heart pumps faster to get the blood to the muscles and the breathing is more pronounced to oxygenate this blood. This is called aerobic respiration. If the heart and lungs can't produce enough oxygen, then the muscles use carbon dioxide instead, called anaerobic respiration, which produces an 'oxygen debt'. This process also produces lactic acid (which is felt as cramp) which needs to be broken down once normal, aerobic, respiration resumes. Once you've stopped exercising, your lungs and heart still need to work as if you were exercising until all the 'oxygen debt' has been paid off, and acid broken down. Although you may have stopped exercising, your body still needs to behave as if it hasn't. If you're fit, then you can go longer before the muscles swith respiration modes so less 'debt' is created and acid produced. Therefore the panting after exercise is short. If you have poor circulation (excess weight, clogged arteries through smoking etc.) then the muscles will switch modes earlier, creating a bigger 'debt' and more acid which will take longer to pay off, so the panting continues longer.

As children, we breathe as we are supposed to... diaphramatically, but for reasons that experts can't explain.. we stop breathing this way around the age of 4 - 5, and begin taking "shallow" breaths. Lay on your back, put your right hand on your stomach, and your left hand on your chest... okay, breathe in through your mouth for 5 seconds.. pushing your stomach toward the ceiling, and then let your chest raise... hold for 5 seconds... exhale slowly through your mouth for 5 seconds... letting your chest fall, and then your stomach. Similar to filling a water bottle... from the bottom to the top... and emptying from top to bottom. Watch a baby breathing, you'll notice that this is the way they breathe, and do these breathing techniques... it will help.

Your propably doing way to much. Cut down and increase it slowly.

Probably because you are out of shape! The more you workout, the easier it will become.

maybe you overdid it too soon...