Why does LO wake up screaming from naps?

And it's every single time. Actually, now that I think about it, he never wakes up quietly, he wakes up once or twice at night and he cries every time, sometimes it's just a whimper but naps oh goodness! It's top of his lungs kind of scream. When he was smaller sometimes he would cry in his sleep but he would continue sleeping, but now he's wide awake, I've let him cry for like a few seconds to see if he goes back to sleep and he doesn't. Any thoughts? TIA