Something strange happened to me last night.

I am 6 feet tall, and weigh 275 lbs and yesterday was day 3 of the Keto Diet. I tested my urine and I was definitely in Ketosis. Throughout the day I had, in total, about 3 liters of water. I did not work outside at all, spent the day inside doing what I always do at work (retail).

My wife and I were outside and I started feeling light-headed. I stood up slowly and walked inside. My wife instantly said that I looked extremely pale and apparently I fainted, because the next thing I know is I'm on the floor and don't remember anything. We wait for my mom to come over to watch our sleeping toddler. While we wait I eat about 10 Goldfish crackers and drink some more water. We go to the emergency room and after a series of tests, they say I was dehydrated and gave me a saline drip.

The doctor said that the diet was okay, but I was simply dehydrated. I'm wondering how much water should I drink!?! Three liters and I was dehydrated? Should I be drinking something besides water?

Edit: When I woke up I used a test strip and I was no longer in ketosis. I'm not sure if this is because of my few Goldfish crackers or maybe there's sugar in a saline drip.

Edit #2: I had good BP, but low heart rate.