As a breeder, I archive my kittens and their weightgain. If I look back, many of my kittens have days they gain little to none. At a little older age (6 weeks) I also have days with weightloss, and it always comes around the day after. I learned not to freak out too fast and look at it logical. If the kitten is playful/active and not more vocal or less vocal, let nature do its work. One day less weightgain is nothing to worry about. As long as they gain, they gain. My alarmbells ring when the kittens act differently and/or lose weight. I only help if it's really needed as mom's milk is best and you want them to continue on that if possible. Nature is very strong, but keep a close eye just in case you are needed after all