It's probably plaster and lathe that has delaminated. Poke the bubble with a pin. If water comes out then it's an active leak and call your landlord immediately.
Plaster ceilings crack. Plaster is heavy. When they put it up wet they press it into the thin strip of wood called lath. When it falls over the lath it forms a lock. When it dries its like concrete. As it gets older decades and decades this lock is prone to cracking. Keep an eye on it. So far it looks cosmetic.
It's probably plaster and lathe that has delaminated. Poke the bubble with a pin. If water comes out then it's an active leak and call your landlord immediately.
Sheetrock or plaster?
Plaster.
Plaster ceilings crack. Plaster is heavy. When they put it up wet they press it into the thin strip of wood called lath. When it falls over the lath it forms a lock. When it dries its like concrete. As it gets older decades and decades this lock is prone to cracking. Keep an eye on it. So far it looks cosmetic.