Question

why do we feel pain and fatigue after heavy exercise?

Answer 1

when we do heavy work out we feel pain & fatigue because when we stress on our muscles than usually this brings damage to our muscle fibres but if we do it regularly we do not feel that much pain
when we do heavy work out we are spending so much energy that we feel
extremely fatigue 

Answer 2

Answer:

When your body is injured in some way or something else is wrong, your nerves (cells that help your body send and receive information) send millions of messages to your brain about what's going on. Your brain then makes you feel pain.