Ow, your little one must be so sore!
Well, you seem to have investigated the possibility of being sick - lack of fever doesn't always mean everything, but better to have no fever! This may be a version of the rotavirus (sp?) or just a growth spurt. IANAD. In my limited experience with toddlers, a spike in eating habits usually precedes a growth spurt (and it is fun to look for the signs when I see them next!) Good that you are trying to get her to take liquids, too. What about mixing rice cereal in with the food she is eating?
I am not a doctor, but this is what worked for us:
This may seem complicated and time consuming, but once you get used to it, your diapering times will shorten and her bottom will clear up.
Diaper rash - what helped my little boy was a 'recipe' from his pediatrician: Stomahesive powder (have to order but OTC, I believe), Triple paste, and powder top.
First, pat wash the bottom (discontinue any wipes, too, at least for a little while. Wash with a soft washcloth with only water.
Then, dry it. I realize you have only a little amount of time before the next diaper, but this is important.
Stomahesive powder - expensive, but it keeps the rash from weeping off any of the triple paste, which allows the bottom to heal. It blows on, and if you are not used to it, you'll get a lot when you only need to cover the rash itself (where the skin is open). CVS (or probably any pharmacy) can order this for you. (It's a surgical item usually, so the pharmacist wanted to know what I was going to do with it! :)
Next, Triple paste - *like cupcake frosting* Sounds a bit much, but that was what did it. Anytime we put on less, it didn't take. Not fun for him. Cheapest place for Triple paste was CVS again (didn't check Rite Aid or others, not local) and at that, the pound jar was most economical.
Then - cornstarch powder (no talcum) - over the top, to keep it from sticking to the diaper. (be careful patting your baby's bottom - if you use too much powder, you'll get a dust cloud!) (WalMart has the lowest on this one).
Well, my little ray of sunshine just woke, so I hope this helps your little girl!