Sleeping patterns

So my LO is 4 months old as of the 6th. At night time when he’s in his crib for awhile (an hour or more) he will start pulling his legs up to his chest and grunting and moves his head side to side. I can’t figure out why he does this he eventually will stop and go back to sleep but does anyone else have this issue? Is it gas? Hunger? Reflux? It’s so frustrating and I hate seeing him thrash around I want to pick him up but don’t want to disrupt him. Helpppp.