Has anything changed in the household recently (like routines, food, new people coming and going, is the roommate new?) It sounds like she is is trying to tell you something. My experience with my dog has been that when this happens, it has either been illness or I have been negligent in getting him out and taking a walk, exercising, or giving quality time. Now that is just my situation.
Females are capable of "marking" just like males (just happens much less frequently). I would venture a guess that she might feel threatened by something in the house or even outside...any new dogs next door, or roaming about? She might be trying to set her territory.
Just my two cents.
Matthew |