Sherry was very work driven. How could she not be? The only reason she was allowed to stop being experimented on was to accept her current job without a fuss. She was usually scared to call in.

But... Okay, she shouldn't fake sick. But she woke up so comfortable. In your arms, being spooned, and she was so warm, and the sun so pretty coming through the window. She just didn't want to go.

So she put on a nice sick voice and called in. She may have made it a bit dramatic, hoping to get her lie across as very believable. Then she hung up and crawled back into your arms.