Nail through ceiling
Looks to me like a drywall screw or nail has "bubbled out". It happens sometimes due to wall movement, especially with weather swings (like a real wet winter or a long drought). It also happens when moisture gets to a nail or screw and causes it to corrode. Hopefully your kinda new roof is not leaking. I don't see any signs of plaster discoloration indicating a leak.

Get a ladder and bang the nail back in (or give the screw a turn) and spackle over it.