I am a chronic carrier of Hep B and take tenofovir daily to keep the HEP virus at bay. I remain HIV negative. I contracted the HEP B even though I had been vaccinated for a couple years earlier!!! No one ever checked to determine if the vaccine had taken, which in my case it had not, and this is common among nearly 10% of those vaccinated the first time!!
As for your contracting it, I would think the fact you are on PReP may be your saving grace. Truvada is 1/2 tenofovir (300 mg) the same doseage of tenofovir I take daily to keep the Heb B undetectable. Hopefully the PReP will work for the Hep B as it will hopefully help with the HIV! Good luck!