3 Answers

Any physical pain in a person's body, is a signal that something is wrong. It may be time to either do some research on your own, see a doctor, or go to emergency.

Cramps are due to a lactic acid buildup in the muscles.  When you fatigue, you do not keep pack excreting it, and it builds up to prevent you from breaking down.  When you catch your breath, really you are removing them and replacing the oxygen levels to allow you to continue, with your second wind, as we call it. 

Some medicines can cause cramps as a side-effect, or make cramps occur more often. These include: thiazide diuretics and loop diuretics (water tablets), nifedipine, cimetidine, salbutamol, statins, terbutaline, lithium, penicillamine and phenothiazines (prochlorperazine, perphenazine, chlorpromazine).
Over-exertion of muscles.
Dehydration.
Conditions that cause alterations in the balance of salts in the bloodstream (such as a high or low sodium or potassium level).
Some people who have kidney (renal) dialysis get leg cramps.
Pregnancy - usually in the later stages.
An untreated underactive thyroid gland.
Peripheral arterial disease (narrowing of the leg arteries which causes poor circulation).
Excess alcohol.
Some uncommon disorders of nerves.
Rare causes include: cirrhosis of the liver; lead poisoning; sarcoidosis.