Being asked about your sexual experiences by cops isn't exactly a comfortable situation, especially when you're a rape victim or have just been traumatized. There are many reasons for her not want to say anything about him, including that she likely didn't want him to involved/to know about it just then and the situation was highly uncomfortable to speak of to begin with. Think about it, would you want your bf/gf/spouse or even friends to find out you were raped by the cops knocking on their door demanding their DNA or would you want to be the one to tell them on your time (if you ever decide to tell them at all)?
A lot of victims never tell anyone until much later and some never tell even those they are extremely close to. When you're raped or traumatized you've just experienced a loss of power and control at the hands of another person. Victims of abuse and rape regain that control and power in strange ways and often those ways are unhealthy or irrational until and unless the trauma is dealt with and the victim provided healthy ways to cope. Withholding information about him/not wanting to answer could also be a way of asserting control over some part of her life, and sex life, that hadn't been violated. People can react very oddly to trauma.
But really, there's a lot of reasons why she would get goofy/scared or whatnot over questions regarding him, including fear of being disbelieved and/or written off by authority figures, friends, family, etc. and just not wanting to talk about it.
