Ali, yep, you have the low carb flu. The good news is that you should be within a couple of days of getting over that (the really crummy part) and in another week should feel pretty good.
It sounds like this is your first time doing low carb. If so, you have in your grasp what low carbers call THE GOLDEN TICKET. You only get this once as a newbie….but you will lose weight like crazy, and it will never happen again. Once your body figures out what is going on and learns to switch over to burning fat and ketones, it will remember that forever. If you chuck it in now and start eating carbs, if you try giving a low carb diet a shot again it will work….but not as fabulously as your very first time. Of course you will not feel like total crap on future go arounds. But that is the price you pay as a first timer….but the rewards are amazing if you can stick with it a few more days. You are almost there.
You might try upping your salt intake a good bit Often the headachy stuff is from a salt/mineral impalance and drinking some salty broth helps a lot.
You could also try eating a bit more so you are not dealing with both the 800 calories and the low carb at the same time. For the first 10 days you will lose enough with just the low carb alone and then could go back to the reducing your calories. Eat something fatty and filling….up your fat intake. Have more butter on your veg. Try a brown butter sauce (cook 1 Tbs butter in a skillet until it turns a golden brown and then add a squeeze of lemon) over some cooked fish or chicken filet.
Just be a little bit easier on yourself……an extra 400 calories right now to help you through this is better than dumping the diet. So do only what you can handle. Its a marathon not a sprint. It is a whole different ball game once you get over the low carb stuff. Hang in there…good luck!