I'm just going to come out and say it -- because you seem to know nothing of breeding you must spay your dog. It will do you and her a huge favor.

As far as humping -- she is not fixed. Unfixed dogs have sexual drive and hump things more often than a fixed dog. Though girls' sex drive is usually only there during heat ... do you know how to tell when a dog is in heat? It is possible she is in heat and you don't even know it. Is her private area swollen?
Or, like BC_Mom said, it could be because of dominance.