there's two scenarios here, either one you didn't buy as much food as you thought you did (however unlikely) or
two (and I suspect most likely), sometimes if you move to the shops in a different tab (or even in the SAME tab) and buy something, it doesn't always acknowledge that it's been purchased. (It also happens with brushes and the potions from Malefix). I believe this is happening because when the page loads now, it loads the inventory first (even for the baby creatures that can't use the things). At which point it stays 'stuck' with that inventory total. Refreshing the page doesn't always help, as you may need to open a completely new window/tab for the creature and then carry on feeding.
The easiest way to check on the older firefox browsers (not sure on the newer ones since I haven't been without food for a while) is that the feeding icon turns pink when there's no food or the creature has been fed already and then make sure you reload the page after checking/buying more if needed. Best of luck with everything here too