A common symptom of electrolyte imbalance is water retention. Once you are on keto, your body doesn’t hold on to salt like it used to when you were eating carbs. You need to supplement salt and can do so by taking around 2 teaspoons (some people take more) of salt a day - preferably pink salt or sea salt as these have a good range of minerals compared to table salt. You also have to ensure you are having an adequate level of magnesium (400g) and potassium (varies from 1,500 to 4500mg - I recommend starting low and increasing as needed) either through diet or supplementation.
Without electrolyte supplemtation or intake, you may struggle with one symptom or the other on keto.
Also, high water intake can deplete you of electrolytes (this can explain your current water retention as you would have lost sodium and potassium through urination and not supplemented them back into your diet). I recommend only drinking when you are thirsty and not overdoing the water.
The rash could be an allergy to something you recently introduced to your diet ( sweeteners, new products etc). Try seeing if you can identify and eliminate these to see if symptoms improve. It could also be your body adapting to the ketogenic diet. See the video below on what might help with that.
https://youtu.be/Od7LYWhcLFs
