This is a very general run down of what allegedly happened, but as others said, the tl : dr is, 15+years of testosterone caused her reproductive organs to atrophy. The atrophy was so bad, her cervix began to calcify into a hard little nub that was totally fused shut against her uterus. (Her overies above had also atrophied and fused agiasnt the uterus)
Now, the cervix is meant to stay open, because of course, periods need to get out somehow. The problem was, Buck wasn't having periods due to the cross sex hormones she was taking, so she had no idea her cervix fused shut.
Even if one isn't having periods, the cervix stays slightly open to allow natural shedding of cells from the uterus. It's not really enough to be noticeable (as in discharge) but it does gradually happen.
Since Buck's did not have an exit for these cells, debre, and dead tissues, over the course of many, many years, they built up and decayed inside her uterus. It was so bad it that the septic gunk backed up into her fallopian tubes and out into her abdominal cavity. Then everything burst. She had to have an emergency hysterectomy and major antibiotics and as others have said, this nearly killed her.