I'm 24 and weight 188 lb i tried exercising i did fine for three days but today my knees started hurting a lot what could it be?

Over doing it: Start small to grow big. Exercise is not an all or none endeavor. It is a continuum. Keep in mind that a little is better than none and you can do something today, so don’t worry about what you will do next month. This perspective is hard for anyone who expects a lot from themselves and sets long-term fitness goals. Don’t expect results overnight. But do expect to take small steps every day.