It's really great that you noticed the change and are checking up on it early.

I agree with Barb that she sounds like she is doing very well. I think you would have noticed if she was dehydrated. The wee gets very dark and smelly, and particularly in a cloth nappy I think you'd know. Other signs that you can look out for are the skin and eyes. If she has bright eyes and her skin tone is good (the skin should bounce back when pinched) then she's fine.

It might just have been her using more in the heat.