Susan first came to the clinic complaining of vaginal bleeding. Gregory House informed her she had been pregnant and had suffered a miscarriage. However, Susan told Dr. House she had not had sex in over a year. Dr. House didn't believe her and when she insisted, Dr. House suggested she start her own religion.
Later, she returns to clinic to complain about a strange spot on her neck. House diagnoses a hickey, but Susan denies having sex since her husband and she had divorced over a year before and even though they live in the same building, she ignores him every time she sees him.
Later she returns with a rash. House tells her it is a carpet burn. Susan becomes angry. She admits that she loves sex and misses having it, but strongly states that she hasn't been sexually active in over a year. House finally offers to research her brain activity while she is sleeping. It turns out that Susan is sex-somnambulist, and she unknowingly has been visiting her ex-husband while she was asleep, and having sex with him without even realizing it firsthand, due to her Sexsomnia (the apparent hickey and carpet burn she had were received from having sex). They live in the same building, and the fact that she hasn't had sex with anyone else in over a year since the divorce, apparently caused her stress, resulting in her having sleep sex. This explains why she was pregnant, because she was having sex with her ex-husband in her sleep. Neither of them wore a condom during their unintended lovemaking, thus, they ended up conceiving their unborn child together as a result. This also explains why her ex-husband thought she was sending "mixed signals".
Susan promises to kill him, but House tells her that there is no way her husband could have known she was asleep, since sexsomniacs act pretty normal, which convinces others that they're in control of their actions. Its more likely that she has unresolved issues with her ex-husband that she has to work out with him.