I would try to cordon off areas of the apartment temporarily to see if you can isolate it to something in one room. For example, close the bedroom door and seal under the door with towels and try sleeping in the living room for a night. Alternatively, try sealing off all the bathroom/sinks/shower drains for a day.
You don't want to move and bring the problem with you, (like a memory foam mattress or pillow giving off gases) so you have to try to rule out your possessions....by keeping them away from you while you test for a day or two.