What is their diet like, have you changed it lately and are they getting a decent amount of greens?
Could be a bladder problem, is she having any problems weeing or round her back end, does she seem uncomfortable/restless/ irritated or more sleepy or any other symptoms? Is she keeping herself clean and are her eyes bright and general character normal?
May be she's bored and playing with the bottle, does she have things to keep her entertained?
If in doubt ask your local vet for advice or take her in for a check up.