Question

why cramps are formed in our legs after vigorous exercise

Answer 1

This happens because when you exercise vigorously, the amount of oxygen that your body takes is not sufficient to be wholly absorbed by your muscles, they need more energy so that you can exercise well and hard. But they don't get oxygen that creates lack of energy. So your muscles start breathing themselves and they start accumulating carbon di-oxide, which when happens, produces lactic acid and this lactic acid creates pain 
it can be treated by breathing as deep and as long as you can. or if doesn't help you, you can take a hot water bath..

Answer 2

It is due to after vigorous exercise the oxygen is not distributed equally to all parts and so there is over secretion of lactic acid which causes cramps in our body.