Sick 19 month old

My daughter is almost 19 months and started daycare a couple months ago. Ever since she's started, she's been battling cold after cold. On Thursday, she woke up with a fever so I kept her home. I took her to the doc and they ran a strep, flu, and UTI test...all negative. They said it was just a virus and it would have to just run its course. Her fever got up to 103.4 Thursday evening but we were able to get it down and keep it down. Her fever broke Friday afternoon, but now it is Sunday and she is still not feeling well. No fever, but very lethargic, barely eating and not drinking very much. It's been a little better today, but I'm still super stressed. Is this just how viruses go? Do they last this long? She's definitely lost weight which worries me. I'm also worried about dehydration and believe me, I've tried everything to get her to drink more/anything. Any advice/suggestions would be greatly appreciated.