One common parasite is giardia and sometimes it is hard to pick up with standard tests. Some individuals pick up a bacterial infection in the stomach called helicobactor pylori when traveling and this can cause fairly impressive nausea. Sometimes we perform endoscopy of the stomach in patients with persistent nausea. We can also rule out a more unusual process related to foreign travels called tropical sprue. I do note that many people do not feel well after returning from extensive travels and sometimes we aren t able to find out why.
People eventually recover on their own but it can take awhile.
This is not a reason to be checked out thoroughly. However, I would try not to get too discouraged. I hope this information is helpful to you. Good luck with your problem. If you are in the Detroit area, we would always welcome to come see us.