It could be environmental. Perhaps you can ask your doctor to test you for toxins, such as molds and heavy metals. Dr. Shoemaker has a lot to say about mold toxicity and "sick building syndrome." www.survivingmold.com
You could also be reacting to some kind of chemical in the house. Chemical sensitivities have been observed in reaction to carpets, drywall, and many other construction and finishing materials. The hard part is figuring out what. It can take a process of elimination to figure out the cause.
Sometimes a yeast problem develops when the system is out of balance. You can look up more about yeast online and in articles about a book called The Candida Diet.
The nausea could also be connected to the terrible headaches. I have had only a couple migraines in my life, and both involved nausea, one left me vomiting. Have you seen a neurologist before?
I'm starting to think my house might be making me sick. My daughter has had similar symptoms this week and this is the first full week we've both have been home in a long time. (Not working right now).