What could cause instant nausea when entering larger passenger planes?
There are a few things that could be causing it but chances are its not the aircraft. When on the ground, with the engines off, aircraft simply have exterior air circulating through them. In the hot weather they are often connected to an external AC unit similarly in the cold weather they are connected to a heater. Cabin smells tend to vary widely and I cant find any data other than anecdotal evidence about how they are controlled and if they are quantified. 

Once the engines have started they can use bleed air but if your symptoms are calm after 5 minutes its likely they are still using ground fed air at that point. 

You may be experiencing light symptoms of Claustrophobia upon knowing you are entering a small, enclosed space. Some evidence points to the fact this can be cured with repeated exposure so the more you fly you may notice the effects reduced.