Girls, please tell us ?we've been on mixed feeding for two weeks now, …

Girls, please tell us ?we've been on mixed feeding for two weeks now, we give her formula 1-2 times a day, I noticed when she sucks on my breast she sweats on her forehead (I think she's losing fluids... maybe it's worth offering her water? I know that babies under 4 months don't get water (( but we're on mixed feeding ( I'm afraid that then her tummy will hurt)) And I'm also afraid that she might be dehydrated ?who has experienced such a situation ????