Without an intention to scare you, I would like to share the facts. Unprotected Bottom anal sex is the riskiest of the risky behaviors and must always be done with protection using plenty of water based lubes.
If it was an HIV Duo (Ab+Ag) 4th generation test, done after 4 weeks of your exposure, you are fine and your test was conclusive. If not, please repeat it after 28+ days to get conclusive results.
Lesson learned, Never Ever unprotected again with anyone, outside your mutually committed monogamous relationship.