My knee is always pain in the next day after i go through some intensive exercise. however, the pain will cease after two days. what's wrong with it?

More info: This cannot be answered without additional evaluation including location of pain. Best idea is go to your doctor for an examination. Afterwards, treatment recommendations can be given.

Nothing.: In exercise, form is very important. Proper form prevents excess strain and injuries. In a lunge, for instance, you do not want your knee to extend beyond your toe, as this will cause knee strain/pain. Because it happens repeatedly and is rapidly better, it's unlikely to be a bad injury. Depending on what you are doing, it's likely simple over use or bad form (or maybe you need better shoes).