I'm going to try to plumb my library for this, but I think I read it about 4-5 years ago (around 2014-2015) as a free Kindle book. The primary protagonist was a woman who had vague memories of having been at a bar attacked by vampires and, grievously injured, staggering off, only to wake up chained in a cell. She's forced into a series of fights (I don't remember if they were against just humans, or against supernatural creatures as well). At least at first, all of her opponents are female, and larger than her. I only have vague memories of the fights themselves other than one involving a bear hug with either the spine breaking or the target thinking it was about to, and another involving the knee being targeted with kicks. I don't even really remember who was attacking who in that case.
At least one of her captors is a werewolf, whose blood she occasionally receives, making her a much more effective fighter (this sort of doping was not really an accepted practice, and was being done on the sly). I believe that there is also a vampire on her fight team. Humanity, as a whole, was unaware of the supernatural among them. She had a romantic relationship (with overtones of Stockholm Syndrome) with one of the people on her team.
I don't think that I finished the book, but I remember she'd won at least 2-3 fights, and was picking up bits and pieces of her past, realizing that she had given birth to a child at some point, and I think suspecting that she'd had a husband or boyfriend who died at the bar when the vampires attacked.