Within the canine world, mounting behaviors aren't exclusively for the boys -- female dogs also exhibit them. All dogs are potential mounting candidates; the behavior is, generally speaking, totally normal. But it also can be associated with problems such as anxiety or medical ailments.
Sexual Behavior
You cringe when Fluffy attempts to mount your next-door-neighbor's cocker spaniel; for her it's totally normal, even in a girl. If your dog was spayed after going into heat for the first time, it may have been enough time for her to take on hormonal habits, including humping. Dogs fixed at later ages retain sexual behaviors -- such as mounting -- after spaying or neutering, at least for a time.
Emotional Overload
If your female dog is overstimulated, she may mount another dog. If a close pal of yours visits your home and just happens to bring along her new doggie, the unfamiliarity, confusion and sheer wonderment of the whole situation may just turn your pet into a nervous, bumbling and mounting mess, at least until the fanfare dies down. This behavior isn't hormonal or sexual -- just very, very enthusiastic. A lot of mounting behaviors have absolutely nothing to do with sexuality.
Time to Play
Dogs without a lot of experience interacting and playing with their peers sometimes partake in mounting behaviors during play sessions. If your cutie was taken away from her mother and littermates at a very young age, chances are reasonable that this may be the case for her.
Social Ranking
Although not very common, dogs of both sexes occasionally mount other canines as a way of expressing dominance and elevated ranking. Perhaps your pooch is feeling threatened and territorial about the idea of seeing a new dog on her home turf and is attempting to show the other that, in this territory, she runs the show.
Compulsion
Mounting, especially when excessive, can point to a compulsion, similar to obsessive barking or running after shadows on the walls. Compulsions frequently pop up as responses to anxious and frightening situations in animals, so you may want to investigate exactly what may be so troublesome in your dog's life and fix it. If your dog is stressed and confused about changes in your household, she may react by developing a compulsive behavior such as humping. Since compulsions can be disruptive to a dog's well-being, they're not something you should ignore, whether through consulting a qualified expert in canine behavioral problems or simply by giving your cutie a little bit of extra love and attention during a tough period of transition.
Medical Ailments
Although humping behavior in female dogs is in most cases normal, play it safe. Set up an appointment with the veterinarian to make sure that the mounting isn't connected to an overarching health issue such as allergies, urinary tract infection or anything else.