If you are experiencing pain while running then immediately stop. Try to find out why is this happening and how many times it has happened before. If it is happening frequently something is wrong and so it is better to call a doctor or vist one. You might be having a diabestes or blood pressure problem.
So better get yourself checked by your family doctor.

The pain during running can be cause of various reasons and so it is better that one goes to the root of the problem. The most common running pain is when one gets it on the knee. Many atheletes and runners experience knee pain and they cant escape it. You probably have runner's knee if your knee cap hurts when you walk or run, particularly when you walk down stairs; and it hurts a lot when you push the kneecap against the bone behind it. It usually does not hurt to pedal a bicycle.

I tried running one day, but it did not suit me for two reasons. One is that it pains a lot in the joints of the legs and the other reason is that while running I tire easily. So I avoid running as an exercise and I do more of cycling and swimming. They are equally good options.