I read someone's (true) account once of having been raped at knifepoint in her kitchen. She noted that later in life when she thought of it, it surprised her that she had a sexual response to the memory. (Except for being raped, she had not been wounded. She associated the knife with the forcefulness, and with her not being responsible for the sex, but not with pain.) Apparently sometimes even an undesired event with a sexual component can later become a vivid sexual memory.
As her boyfriend, I would believe what she said (i.e., that it happened). Since she survived and didn't get shot or brutalized other than being forced to perform oral sex, maybe the indelible nature of the memory has tied itself in her mind to the sexual aspect of what she was forced to do.
Just be supportive. Ask her if she wants to see a therapist about the trauma, or if she wants to look into trying to report the crime at this late date. Then just be there for her.
It sounds like she was sharing a fantasy, Joerezz. This is not a completely unusual fantasy, but it is an unusual situation to happen in real life. I think I'd try to ask in a nonconfrontational way if this was a fantasy she wanted to act out.