Seconding what Nat said about water--deep purple means you aren't drinking enough (or let your calories slip too low, which is gonna cost you metabolic oomph down the road!)

However, you may have also run into a food sensitivity--think about what you ate the day before you got the drop in ketones. Was there a new food you added? A single sugar free candy, 3 g carbs, with its sugar alcohols, will toss me right out of ketosis. So will wheat in any form, even a bite. If you have discovered a food sensitivity, stay away from it! (hint--it's usually the foods we love the most) It may cause bloat and trigger cravings, too.

HTH