Help my son won’t sleep without crying

My son was doing so good at nap and bed time. I give him his pacifier rock him and he would be out in minutes. He has been giving us the hardest time with naps and bedtime. He will go to sleep but as soon as he hit the mattress or just when we think we made it out of the room and close the door he is standing up screaming bloody murder and crying !!!!!! HELP!!! This isn’t fun at all. Why is he doing this !