Pretty simple sentence: a hoodie is a top (usually a sweatshirt) with a hood. The hood being a round cap-like piece of fabric that covers your head.

They went to sleep and that very same piece of fabric got jostled underneath their back and got stuck! The fabric, now constrained by a good portion of their body weight, either applied a great amount of pressure to a very small area of their body or caused them to get stuck in an unnatural sleeping position.

Either could conceivably lead to considerable localized pain.

(And I assume they don't know for sure since they were asleep as this occured)