In my house Sunday is usually pizza night. I never had any problems joining my family in eating pizza once a week. Over the past few months though I noticed that the smell of pizza makes me nauseaus. I would try to eat it and feel like I am going to throw up. It now has gotten to the point where I can't even think of pizza without being nauseaus. I am nauseaus just writing this.
1.) You are simply sick of the food. You are sick of having regular pizza nights.
2.) Your body is done with pizza for now; it needs a break; it has created a "food sensitivity" (like a very subtle allergy) and that's your body's way of telling you to take a break from eating pizza.
Nothing to be too worried about. Also partially a mental thing; just don't dwell or worry on the pizza thing.