One of the things that can cause nausea is the push pull from trying to do CBT.
Cycling from positive to negative and back again. An example Is saying "I'm okay every thing is all right". Things settle down and then you go, " no I'm not, I feel horrible" muscles tighten up, the lining of your stomach gets inflamed and you feel horrible. So you do the self talk to calm but in creeps the negative and the cycle starts again. Muscle relaxation will help as will box breathing and anything to keep you calm. Tums or something to reduce acid in your stomach will probably help and the calcium is calming.