Can I be immune to HIV?

I used to be addicted to intravenous drugs, and was involved in risky behavior with my boyfriend at the time, who did not tell me he had hiv and hepatitis c until 4 months after the relationship started. I had unprotected sex with him, AND shared needles frequently with him. I had contracted hep c, but 1 year later my body had fought it off without the need for medication. I am now std free. I had never tested positive for HIV even after multiple screenings. Does anyone know why?