I think it might have been due to the cleansing and healing properties of the wheatgrass juice, it might be possible that your body needs some detoxing and or healing.
Wheatgrass usually gives people more energy, it has many vitamins, minerals & has alkalizing & detoxing benefits but if your body is overly acidic or has a lot of built up toxions, it can cause the body to feel ill or nauseous
because the alkaline properties will neutralize the acid in the body while removing toxions while healing parts the body.