Vamps

Stacy (Krysten Ritter) and Goody (Alicia Silverstone), are two socialite vampires living the good life in New York City. Goody was turned in 1841 by the vampire queen Ciccerus (Sigourney Weaver). She struggled with her life as a vampire until Stacy was turned by Ciccerus sometime during the early 1990s. Goody was able to teach Stacy how to use her new abilities, like sustaining themselves on rat blood, but keeps her actual age a secret because she is afraid of being viewed as old.
While at a vampire meeting, Stacy explains how she struggled with drug addiction as a human, but is now happy since being a vampire gave her a second chance. Goody also discovers that if their maker or "stem" Ciccerus is ever killed, they would revert to their human ages. While working at a hospital as exterminators, Goody runs into her ex-boyfriend Danny (Richard Lewis), whom she has not seen since the 1960s. They re-connect under the pretense that she is Goody's daughter, but Danny eventually learns the truth when he sees her bite into another man to prevent a stroke. When he asks why she left him, Goody explains that even though she loved him, she did not want to stand in his way of finding someone he can actually build a life with.
Meanwhile, Stacy begins a relationship with a young college student named Joey (Dan Stevens). It is soon revealed that Joey is the son of the infamous vampire slayer Dr. Van Helsing (Wallace Shawn) who is in town to find and kill Ciccerus. After spending the night at Joey's place, he sees Stacy crawling down the side of his apartment building in order to get home before the sun rises. Despite his initial shock, Joey accepts that Stacy is a vampire and the two resume dating. When Stacy begins to experience strange vomiting, Goody discovers that Stacy is actually pregnant, but that the baby won't survive unless she becomes human again.
Goody tells Stacy that they must become human again in order for her to stay pregnant, and the two decide that while they have had a lot of fun as vampires, "being young is getting kinda old". Goody then goes to Dr. Van Helsing to get his help in killing Ciccerus. When asked why she wanted to become a vampire, Goody explained that her husband and sisters had been killed during a cholera epidemic, and that she had to stay alive in order to protect her two children. Van Helsing then has his assistant look up Goody's living descendants; most of them being doctors, teachers, and politicians. The girls team up with Joey and Dr. Van Helsing to track down Ciccerus and after a struggle, they end up killing Ciccerus in order for Stacy to keep her baby and have a future with Joey. Stacy looks relatively the same despite being forty years old, but Goody rapidly ages into an old woman. Revealing her actual age, Goody accompanies Joey and Stacy to Times Square where Goody reminisces about her life. As the sun rises, she disintegrates into ash.
A few years later, Stacy and Joey show up at Dr. Van Helsing's house to pick up their young daughter, whom they have named after Goody. As Van Helsing plays with his granddaughter, he notices that the little girl sports a set of vampire fangs.

Question: Why does Stacey experience strange vomiting?
Answer: She is pregnant.