If there is no milk involved, it is not from lactose intolerance or something like that. What is the mechanism for caffeine causing nausea?
There are some conflicting results concerning the effect of caffeine on nausea.
One study 1 reported that caffeine withdrawal is associated with nausea and vomiting which might suggest that caffeine might reduce nausea.
However, two years ago, a study 2 was conducted in postoperative care to reduce nausea and vomiting after general anesthesia by administrating caffeine. Interestingly, patients in the caffeine group experienced more nausea than patients who received customary antiemetic prophylaxix. This would suggest a potential pro-emetic contribution of caffeine.
So, current evidence regarding the effect of caffeine on nausea is sparse (and somehow conflicting) and further studies are needed to investigate the effect of caffeine on nausea.
Edit (regarding the possible cellular mechanisms of caffeine causing nausea)
I haven't found any study investigating the effect of caffeine on nausea (ie the area postrema). The last study conducted in the postoperative care (and showed that caffeine increased postoperative nausea) did not suggest any pathway or mechanism either. So further work is needed to investigate this possible association.