Honestly, I think it's just stress, and your body's physical reaction to the emotions you feel, after thinking about your abuse.
However, it would be wise to get checked out by a doctor to make sure there is no physical reason for your nausea.
It's your body's normal fight or flight response. I'll try to put it as simply as possible but once adrenaline is released, your body sends every resource it has to to the heart, lungs and brain (to enable you to either fight the threat, or run away from it). The body will simply 'shut down' parts that aren't needed at that time. Which includes your digestive system, which means it will literally eliminate any thing or any functions you don't need. Hence why you may feel nauseous, when anxious. But also why people vomit, or loose control of their bowels or bladder when really frightened. It is the body's natural response to the hormones that get released when we're afraid - literally so the body can put all it's energy into running away or fighting the threat.
I'm having similar issues at the moment, with feeling nauseous pretty much 100% of the time. But also to the point that if I do try to eat I vomit.
I've always been a comfort eater, so this is a completely new experience for me. But after several new trauma's over the last few weeks I simply can't eat. However, I'm overweight, so I'm not overly concerned at the moment. I've lost about 3 stone in quite a short space of time, but I'm still in no danger of wasting away. I also have a couple of broken ribs, which are on the mend, however vomiting with broken ribs is immensely painful, which is also putting me off even trying to eat at the moment because I'm scared of the pain involved with vomiting.
Honestly, I think the only way for me to overcome this nausea is to sort out my head - sort out my PTSD. The only way I know to do that is to go back to therapy to address my recent trauma's. . . . . sort out my anxiety and stress . . . . thereby hopefully reducing the nausea. However that's quite a long road.
I need to earn some money, to pay for therapy, so in the mean time I'll just keep working when I can to bring some money in. Also, my broken ribs are healing, and then maybe at least trying to eat won't be such a problem, because in a week or so hopefully if I do vomit, it won't hurt like hell.