When i got my nipples pierced, the second one hurt more than the first. Why? I conclude it was because I had no previous sensation to which I could attach the experience of having a nipple pierced. By the time I had the second one pierced, I had a frame of reference about what sensations to expect and found myself flinching before the needle even touched me.
In many ways I think the psychedelic experience is similar. In the beginning, I had to frame of reference to say "This is comeup anxiety" or "My stomach gets queasy on the comeup." As we collect a wider range of experiences, we sometimes subconsciously project our expectations into our future experiences.
i.e 2c-e made me queasy last time --> I expect to be sick during first 2 hours of future experiences = psychosomatic nausea during future experiences.