Respiration in Organisms

Paheli participated in a 400 m race competition held at her
school and won the race. When she came home she had mixed
feelings of joy and pain as she had cramps in her leg muscles.
After a massage she was relieved of the pain. Answer the
following questions related to the situation.
(a) What can be the possible reasons for the pain in her
legs?
(b) Why did she feel comfortable after a massage?