Sheila was first seduced, then kidnapped by Spike, then given to Drusilla. Drusilla needed to be fed as she was too weak to hunt for herself.
Before she was turned, she had a reputation for being wild, which she confirms when we see her leaving the Bronze with two unknown guys seeming somewhat drunk.
After she was turned, she tried to trick Buffy pretending she was still human. However after the attack fail and she see one of her fellow vampires get staked. She wisely runs off, never to be seen again.