Nausea can occur for several different reasons, most likely not nutrient deficiencies, however one symptom of magnesium deficiency can be nausea. I would talk w/ your Dr. about getting blood work if you think you may be lacking certain nutrients or you can speak with an RD in your area who can assess a daily food log.
Treatment for nausea depends on the underlying cause and usually resolves by itself. Be sure to drink plenty of fluids and if vomiting then replenish electrolytes and drink even more fluids.
Regarding chocolate cravings, you are not alone with that. We actually have a healthy chocolate drawer that everyone tends to raid in the early afternoon here at our office! This could be out of habit or just your body's reaction to low energy and it needs a pick me up. Eastern philosophy says it may be your body's way of telling you that you need specific nutrients - chocolate/cacao are great sources of magnesium, iron, phosphorus, copper, manganese, zinc, and potassium.
Try using cacao to make a chocolatey treat since this doesn't have added sugar, then there's no guilt :)