6 mos help!

Our 6 mos old son is having some pretty awful nights and I’m not sure what to connect it to. We did a form of sleep training at the 4 month regression and he was down to one early morning feed and right back to sleep, or even sleeping through the night. About three weeks ago he started having random night wakes where he is scream crying and nothing will settle him. Two nights ago it was from 11 pm— 5 am straight. He will not nurse, won’t settle in my arms, and thrashes around- for HOURS. I thought it was teething but even Tylenol did not settle him. He falls asleep independently, naps beautifully (2/2.25/2.5/2.5 with 3-3.5 hours of daytime sleep) however, on the nights this doesn’t happen he will sleep through the night. I took him to the dr to check for ear infection , etc and everything was clear. To note, his eyes are often closed when he’s scream crying but I know he’s too young for night terrors. We could re-train but he gets more upset when we do check ins and after letting him cry for an hour without any sign of settling and just getting more upset it seems something else may be going on. Especially since he can make it through the night other nights. Thanks for reading! Has anyone experienced anything similar?