I too used to drink alot of Diet Coke. Started Atkins on Jan 19th, no more Diet Coke and by Jan 20th felt awful. The day got worse and the headache was a killer. I got headspins while my boyfriend was driving and I made him pull the car over. Ate a $20 dinner, went back to my hotel and threw it up, what a waste of money. I knew Doc A had said withdrawal could be strong, he even said the stronger the better 'cuz you're getting junk out you don't need.
Slept like a log, and felt amazing on Jan 21st. No headache, no bad tummy and no cravings.
Since then, I've been in Ketosis every day and have lost 21.5 lbs. That's my 2 cents. If you try it, it'll be rough, but hopefully you'll find a rainbow on the other side in just a day or two.
Elaine