Memories change, then the body

The plan of the entity to split itself was flawed, as all it's offsrping were affected by the Hellmouth. Willow was the first to feel it's affects. She watched her memories shift. There was her first practice kiss, with her old friend Jessie turning into the night she lost her virginity. The Halloween where she turned into a ghost... she was only wearing that outfit because Snyder said that she couldn't wear her selected outfit around kids.

She was distracted from the scenes by a tightness in her chest and a tingle between her legs.