Perhaps it's the pressure to be the perfect girlfriend to Gladys, or simply trying to keep her safe from everything that could possibly harm her. Nina told Gladys everything when she went back in time, she told her how she died and that they actually fall on love in the future. Gladys was amazed by everything but one thing she couldn't denie was how she felt towards Nina, how she was scared of saying anything to her. Nina assured her that she feels the same way and they left the plantation to start their lives again in the deep parts of Virginia where Regina found them.
It was all perfect and love, exactly what Nina wished for and she felt complete. This completeness brought the deepest need for her to protect Gladys and make sure she's not sad nor in danger. And this included giving up the thing she craved the most, blood.
