Why is it that you don't feel pain until hours or sometimes days after doing something strenuous?

Answer 1

Often this is due to the adrenaline in your system, or the endorphins that your body releases while you're exercising or playing. Once the endoephins and adrenaline wear off is when you feel it. Not only that, but doing strenuous activity burns away lactic acid in the muscles. Many times what you're feeling is the lack of lactic acid, the weakness, etc. One of the ways to combat soreness is to drink orange juice, or take vitamin c supplements DIRECTLY AFTER exercising.

Answer 2

The pain is caused by a buildup of lactic acid in your muscle tissue. This happens when you work muscles that aren't normally used. If you exercise the muscles again the lactic acid will work it's way out of your muscles and the pain will go away.