Its possible that you have picked up a bug, hence the nausea. If it goes on for another 24hrs I would pop to the docs.
As far as the half, I think you have explained why you had such a tough race, If you had an awful week beforehand this can affect you mentally and the fact you hadn't hydrated enough or eaten enough........
Get yourself checked out if you continue to feel ill and just write off the half as a race that didn't come together on the day, and move onto the next one.