Looks to me if could be water stain. does your bathroom have a vent in it?

Sheesh, it looks like someone is peeing on the ceiling. Is it a stain or does it wash off with soap and water? If it washes off, it's probably just steam condensation and the yellow color might be nicotine stains or some kind of chemical, like spray deodorant or hair spray, that is getting condensed by the steam.

Looks like nicotine being washed off by an extremely high moisture content that's literally condensing on the cold ceiling and following the slope.
1) quit smoking
2) wash walls and ceiling
3) get an exhaust fan and use it.
4) shorten those 90 minute steam showers to 25 minutes.
;-)
Casey

What the others said. The streaks from condensation can also be seen on the wall and door.

Is there even a shower or tub in there to create condensation?

How long have you been there, how long since it's been painted, etc.?

Oh, also, ewwww. :)

Really doesn't look like sap or mold though!

This could be nictotine gathering in humidity of the bathroom, or it could be latex in the paint. Both have been proven to have this result. You can get a mop and see if it comes off with some Pinesol or sugar soap, then wait for it to fully dry, before putting on a coat of Kilz or Zinzzer mould killing primer. To stop mould growing back, you could try a full white vinegar on the mop after you have cleaned it, and before priming.

I had the same thing occur in the master bath of my 1950s vintage house a number of years ago. The original bathroom fan was not great, and I concluded that it was some form of mold. I scrubbed off as much as I could, applied Kilz and a new cost of paint, and put in a more efficient ceiling fan. That seemed to fix it.