If you believe you are at risk for HIV (have engaged in high-risk sexual activities, shared needles, come into contact with the blood/body fluids of someone known to be HIV+), get an HIV test right away (if it hasn't been 3 months after your last risky encounter, it may not be definitive, so you may want to wait that period out, so you don't have to get tested twice). The flu shot won't interfere. Over a decade ago, flu shots could lead to some cross-reactivity on the ELISA. However, all ELISAs that turn out positive are confirmed with a Western Blot, and those who were HIV-negative but had had flu shots did NOT get positive Western Blots (a positive ELISA and a negative Western Blot means that a person is HIV-negative). Incidentally, the tests have been improved, so the flu shot should no longer even cause a positive ELISA.
Hope that helps. Please ask if you have any questions :-)
I hope your test turns out negative (if you are having symptoms that you think may be caused by HIV, you shouldn't worry unless you've actually put yourself at high risk. Most people with constitutional or non-specific "HIV symptoms" turn out to be HIV-negative).