The animation goof in the movie. The animation goof has Nausicaš's dress mysteriously change completely blue when another animation sequence is shown. However, during that split second, the Ohmu managed to bleed so much it dyed her dress entirely blue (When before it wasn't shown as instantly turning it blue) and even managed to get on her back. The Ohmu bled that much.
It's not an animation goof, it's deliberate. The baby Ohmu was deliberately shown spurting blood like a fire hose for precisely this reason.