Firstly, you should understand that the nausea, vertigo and headache, etc are all perfectly normal, and almost certainly symptoms of detox. However, if you want to avoid these unpleasant side-effects of fasting, you MUST regularly perform the 5-step elimination cycle. This is not an optional add-on, but practically essential for safe fasting, so ignore it at your peril. If you don't know what it is, try reading about it here:
Secondly, you would have been in full ketosis by day 12, so it's doubtful that the small amount you have eaten will have had a material effect on your ketogenic metabolism. Having said that, I prefer to err on the side of caution. In the spirit of Srinath's advice, you should now eat a small amount of fatty food to ensure uninterrupted ketosis. A small amount of soft cheese (say, 1oz. or so), an egg yolk, a spoonful of (unsweetened) peanut butter or healthy palatable oil, etc., are all suitable for this purpose.
As for putting the weight back on.........no, that ain't gonna happen on such a small amount of 'food', unless you now find yourself incapable of resisting the rest of that jar of peanut butter, etc.