What to do if in pain the next day after exercise.?

It depends: If this is just mild pain at this site of one the muscle groups being exercised, then that isn't too concerning, and you feasible could work out the next day. If you have moderate to severe diffuse muscle pain, or severe muscle pain at a muscle group that you exercised you may need to seek an evaluation by your doc. It's possible to have developed concerning muscle damage(rhabdomyolysis).