5 month old twitches when falling asleep

He never used to do this and it’s only with naps when falling asleep. I will watch his arms start to drop and it’s like the startle reflex all over again. And it wakes him up at about the 20 min mark. So if i don’t run up quick enough to rock him back asleep then his nap is over. The twitches are like what we do when we are falling asleep but I’ve never seen him do it before up until this last week. He would usually do a half hour nap in his crib now I’m lucky for 15-20 min before i have to go run upstairs. Has anyone’s baby done this?? I just don’t know how to help other than hold him for every nap