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@absurdbird
It* is *probably water penetration from the shower spray through the wooden cill. The advice to remove the blown plaster is correct, definitely let everything dry out before making good. Check to make sure there is no water penetration from outside.
Is it a Victorian house, with lime mortared bricks and lime plaster? If so, the original plaster is not really a proper substrate for tile in a shower-especially if the shower is used often.
When I do showers in this situation I strip back all the plaster (to bare brick) and replace it with something impervious, cement render, WBP ply or Wedi board (last two options are great if you don't have good plastering skills or lots of time but cost more and need to be properly joined. They also allow for lime mortared bricks to move a bit which they do- a too strong cement render will fight this characteristic and cause problems.
This is all more expense and takes a lot more effort but provides a sound flat plumb surface for tiling and you know its going to last.
But I get paid to do this.
If the house is later (post 30s) or has been re-plastered and you have Gypsum based plaster then patch it up, don't use bonding though as it is very absorbent and shouldn't be used on exterior walls especially next to windows, or in shower enclosures- use cement render (with a multifinish skim if you want). If the repaired substrate is dry, flat and plumb go ahead and tile on top.
I would get rid or the wooden window board (interior window cill) and replace with tile. Tack up a bit of board/ply/whatever to act as a former/shutter, make it level and the same distance or more from the glazing as the vertical reveals, then back-fill with a sharp sand and cement mix (5:1) or you can just use good tile adhesive (I always use Mapei and not just cause of the cycling connection) The adhesive will be flexible, dry faster/take tiles sooner but costs more.
I do this for a living.
:-)