19 month old waking up screaming?

Hi everyone. My 19 month old has always been a fantastic sleeper. He takes 1 nap during the day for about 2.5-3 hours and then sleeps 10-12 hours at night. We have bedtime routine we have followed since birth and even during teething he would always sleep well. Lately though he is waking up from his naps about 30 minutes to an hour early and just screaming! If my husband or I go in there and get him he immediately calms down. We will usually put him back down if the nap hasn't been too long and he will sometimes cry harder or sometimes he rolls over and goes back to sleep. This only happens at nap time. We've also tried not going in there thinking maybe he was just getting too used to us coming in and rocking him but that led to him screaming to the point of chugging for 20 minutes and I couldn't take it anymore and went to get him. Does anyone have any experience with this or advice? I'm hoping it's just a phase but I welcome any suggestions!