I drink nothing else but no added sugar robinsons squash and haven't had any problems with either losing weight or getting into ketosis. Watch the citric acid though as someone said above, don't make the juice stronger than you have to, i find i get used to drinking it weaker and have gradually cut down how much i use just to be on the 'safe' side (if the citric acid effects your keto state, it might make you lose the natural hunger suppression though which is one real keto benefit, as with anything just monitor yourself on it). Anyway, as for feeling ill i'd definitely just say 'go for it' anyway, if this is the only 'wrong' thing you have while sick you will have been doing a wonderful job and should congratulate yourself!