Firstly, it's best not to have unprotected sex with anyone unless they have tested negative three months after their last sexual encounter with someone else, and they are monogomous with you.
Your situation seems good. The negative test in december is encouraging and indicates conclusively that you did not become infected with hiv in september. However, it is not conclusive for the encounter in october. But, remember that a single episode of vaginal intercourse carries a 1-3% chance of infection so doing it just once means that infection is not likely. It's like russian roullette, you can pull the trigger a certain number of times, but eventually you will shoot yourself because there is a 1 in 6 chance every time. Unlikely as infection may be from one episode, this should not encourage you to practise unsafe sex.

Though science has a way of throwing stats and lab reports at us, the human experience is tough to manage. Here's the best advise you can get.

First, are you in a monogumous relationship? Is your partner faithful and committed to this issue? Clear these things up together!

If not, you should take an elisa test 13 weeks after exposure to seal the deal. However, you have the power to literally help eradicate this virus and never take a test again. Simply put, stop unprotected exposure!

My opinion is that you'll turn out fine!