I did a little research on RxList.com. I didn't see anything in the side effects about dehydration except for dry mouth and lips. I did, however, see a warning that it can cause or exacerbate autoimmune diseases. When you mentioned a "butterfly-shaped rash" on your face, I immediately thought of Lupus. Are you drinking enough water? Try using plain petroleum jelly on your face and hands at night, or Almay moisturizer during the day (hypoallergenic.)
However, I do think you should check with your prescribing doctor, especially about the rashes.
Hugs,
Connie