Night terrors or something else?

My 15 month old just started waking up screaming between midnight and 1:00am every night. This has happened for about 5 nights in a row now. Prior to this, he has had no issues sleeping from 7:00pm-7:00am for the past 9ish months. When I go in to comfort him, he doesn't want to be held. He just flails and screams more if I try and pick him up and hold/comfort him. I have just been patting him on the back/bottom until he calms down, but this usually take like 3 times of me going in and calming him for 5 minutes, leaving, he screams for 10 minutes. I go in again and pat him for 5 minutes, he calms down, then I leave and he screams for 10 minutes, etc. Any ideas?!