Kate concussed after the purse-snatch
Concussion is really another word for a bruised brain. She tried not to think about it, that the impact had maybe sent one part of her brain slamming against her skull, and now that part was purply swelling, mashed up against the inside of her skull. The lightpost she held herself up with was what had hit her.
Context
Kate, robbed and swung head-first into a lightpost, narrates her own possible brain damage with the same flat, anesthetized dread she brings to It. The closing sentence is the whole cruel structure in miniature: she keeps herself upright by clinging to the thing that hurt her.