don't know about ftp, but doing intervals indoors, or any sustained hard effort indoors - I'd be really surprised if you DIDN'T feel nauseated...
BITD, when we used to do 'club' indoor sessions on rollers, it wasn;t unusual to have riders almost pass out, heavin chunks was also common for many who ate something within an hour of an indoor group session. And we used to have one of those big commercial stand fans to create some wind.
without the evaporative cooling of riding outdoors, it's easy to put yourself into major heat distress.