If your uric acid was a little high and you cut out the carbs and maybe broke down some muscle (which releases purines), or if you ate a lot of organ meats, herring, shellfish ... stuff high in purines ... you uric acid might have passed the borderline into an acute gout attack? The big toes and thumbs are usually the worst places.
Have some cherries or cherry juice (quite a lot), and eat some dairy food, see if it gets better. If it does, have your uric acid levels tested. Something about dairy protein and cherries tells the kidneys to collect and excrete uric acid. Watch out for beer. It has a lot of a purine in it.