She was marking her territory. Again. Pissing or ******** in the house is just another way for your dog to tell you that she's the alpha and you're not. As #3 said, marking is unusual in bitches unless they're dominants. She most likely considers you a pack member of lower rank. So start showing her who's boss, or she'll be ******** elsewhere in the house to assert her status. Stop acting like a lesser pack member. Use a strong, assertive voice when correcting your dog. Do not lower yourself to her height when greeting her (bend down if necessary, but do not crouch to her eye level). If necessary, gently grab her upper jaw or nose to administer discipline (in a wild pack, greeting involves the more submissive canine approaching on the belly and reaching up to lick the leader's lower jaw, so handling your dog's lower jaw could be taken as a sign of submission to her alpha status). If assertiveness is not working, turn your back and ignore her until she corrects her behavior (she will eventually realize she has done something wrong and run to your front side to regain your attention, so keep turning your back on her until she crouches and whines/rolls over to show her belly/displays other apologetic behavior). Seriously. Take charge, or you'll be her bitch.

A dominant bitch will actually mark. It's more prominent in males for sure, but not unheard of for bitches. Although if she never does it and then suddenly starts, it could actually be a urinary tract infection.