waking up crying

my son is 10 months old and today was the second time he's done this .. he'll wake up from his nap crying , screaming I try to comfort him, pick him up, and he'll just be twisting around crying with his eyes close.. I try to talk to him but he won't stop crying ..it last like 20 minutes till finally he stops and goes back to sleep .. He only does this during his nap too never during bedtime ... any clue what it could be ?