Hi, and thanks for your question.
The window period (the time between when a person comes in contact with a sexually transmitted infection (STI) and when the STI will show up on a test) for HIV and for syphilis is 3 months. In fact, 95% of HIV tests are accurate after just 6 weeks but it can take up to 3 months for the final result. It sounds like you have tested several times after this 3 month window period, which means that your tests will be accurate and that you are negative for both HIV and syphilis and it is not necessary for you to get tested again.
The symptoms you describe do not sound specific to either infection. It does sound like you are feeling quite anxious and it might be helpful for you to see a counsellor to talk about the anxiety you are feeling about this sexual encounter and the fear of possibly acquiring HIV or another STI.
Let us know if this does not answer your question or if you have any more questions or concerns.