It's probably the pain pills.
There are many reasons why someone would have nausea, and to varying degrees, at this stage in the healing process. But the most likely source is excessive pain medications RELATIVE to your body size and how much food you're eating before-hand. Pain meds, such as Percocet and Vicodin should not be taken on an empty stomach. Instead, you should eat some bland, starchy food (such as rice, toast, oatmeal) about 15-20 minutes before you take a pill. This should significantly diminish your nausea.
If it does not, and just in general, you should speak with your plastic surgeon to make sure there's nothing else going on. I'm assuming you don't have fever, chills, sweats or any other systemic signs, in which case it's probably just a question of managing the way you take your pain pills.
I hope that this helps, and good luck,