Hi, there!
I honestly do not believe that your symptoms are related to HIV. For a number of reasons, first of all acute seroconversion usually only happen between 2 and 6 weeks after the infection, and even though a generalised maculopapular rash can be a symptoms, it is never in isolation, as it usually appears together with fever, sore throat, loose stools, palpable lymph nodes, etc. Secondly, your encounter is very low risk; there is practically no HIV risk from oral sex (giving a blow job is estimated at 0% to 0.04%) and lastly, you tested negative with an HIV Duo test after 28 days. You are certainly negative.
Best wishes,
Jose