I am sure you are describing Species (1995).
Some of its plot from wiki:
A government team led by Xavier Fitch (Ben Kingsley) goes forward with the genetic experiment attempting to induce a female, under the (later proved to be mistaken) assumption that a female would have "more docile and controllable" traits. One of the hundred experimental ova produces a girl named Sil, who looks like a normal human but develops into a 12-year-old in 3 months.
Sil first tries to mate with a man she meets at a nightclub (Anthony Guidera), but after sensing that he is diabetic, she rejects him. Unsatisfied, he tries to rape her, prompting her to kill him by puncturing his skull with her tongue. She then tries to mate with John Carey (Whip Hubley), a man she meets after a car accident. They swim in Carey's pool where Sil forces him to open his swimming trunks in order to mate, but he refuses. This act is interrupted by Preston and Laura. She kills Carey, morphing into her alien form, a bipedal mutant with tentacles on her shoulders and back, and flees naked into a forest without being seen by the team.
Here's a trailer: