Just a side thought (may not be applicable to you since your symptoms happen so quickly in response to a dietary change) ... but with me I noticed that when I was on Paleo initially I went overboard with fatty foods (such as meats and oils) and initially this raised my cholesterol levels significantly, which in turn was possibly the cause of me having a pressure-behind-the-eyes feeling and also having worse distance vision. Perhaps when one switches to Paleo and if they go overboard with high-fat foods that can raise cholesterol, this may have an affect on vision if they are not concurrently consuming sufficient antioxidants to neutralise any free radicals. So maybe evaluate your antioxidant intake if you haven't already done so? All the best with figuring it out.