I'd go see a board certified lactation consultant or pediatrician as soon as possible if she's showing signs of dehydration (in the afternoon would be best if you think your milk supply is low at that time). I know that my milk is less in the late afternoon/evening and then it increases overnight. It's also possible that the antibiotic you've been on is decreasing your milk supply. How is your hydration? Are you drinking lots of water and eating well? Have you considered herbal supplementation to increase your milk supply? Please keep me posted about your situation.