It was a love story doomed from the start. The dark Lord wants her and Sirius must give her to him, to protect her. That’s why she thought he didn’t love her, and that’s why Bellatrix Lestrange is the way she is and why she has done everything she has.
They stayed there panting, with him still inside of her. “Tell me you don’t like it now” she murmured in his ear.
After Bellatrix kills Sirius, she writes a letter to him and drops it through the veil.