This same EXACT thing happened/happens to me. Exactly as you described it. And I too use a humidifier.
This is the reason why I wondered whether my lips were still building up a tiny bit and being rubbed off at night. I still kind of wonder but I don't think it's happening. I figured if my lips were building up at all I would notice it more during the day.
Anyways I have tried sleeping every which way and it doesn't seem to matter. I think it's just from having my lips pressed together while sleeping.