HIV cannot be diagnosed by symptoms. Many people who get infected do experience a short flu-like illness 2 to 6 weeks after infection, but many, many more people experience a flu-like illness because they actually have the flu or something similar.
That being said, unprotected sex should only be attempted with someone you trust implicitly. The old saying is old but nonetheless true: when you have sex with someone, you're effectively having sex with everyone else he/she has ever had sex with. Since you are not a monogamous couple and since she is having sex with other men beside you and since you have no way of knowing whether she is playing safely with them, then it is not a good idea to continue to have unprotected sex with her. The reverse is also true, of course: if you have having unprotected sex with other women, then you are also potentially exposing her to any diseases the other women may have.
There's not much you can do at this point except hurry up and wait. The odds are that these flu-like symptoms are just the flu or a similar non-serious virus. You should plan on an HIV test at 90 days after the last unprotected sexual encounter (you can get a test done at 30 days which will be about 95% accurate and may give you some peace of mind).
Another option, which may not be available or affordable, is a PCR test that is usually used to measure HIV-positive patients' viral load. A PCR test now would be able to tell you if there is any HIV in your body (and if it doesn't find any HIV that would imply that there is no HIV to be found). The downsides of this are that the test is not intended for diagnosis and may not be offered, or may not be covered by insurance, or may have a false positive.
Dan