Sorry you are having to deal with all this worry. I can reassure you that HIV is not passed to the person receiving oral sex, so I don’t think you could have gotten HIV from the encounter you described. So, I do not think your rash and headache are caused by HIV.
The window period is the period of time between getting an infection and the infection being detected on a test. The window period depends on the test you get, so you should talk to the health provider who orders the test about the window period for your test. Here in BC, we consider the ELISA test to be conclusive after 3 months. If your clinic advised you to get tested again at 6 months, you certainly can. However, if your only exposure was receiving oral sex, I do not think you could be HIV positive, so please don’t be afraid.
You could catch another STI like herpes, gonorrhea, chlamydia, or syphilis from oral sex, so it’s a good idea to get screened for these infections next time you go to the clinic.
Hope that helps,