Maybe she was trying to cover her bases in case they found any evidence of her in the small bathroom, which they had used (probably extensively) to help in their clean-up/cover-up. So she needed to say she had gone to the cottage to take a shower, so if they found anything, she could say, oh but remember I took a shower there that morning?
Couldn't she say she took a shower there some other day?
BAM! One thousand and one!