my in home rn says that this is totally normal. She told me that before a feed most babies have sunken (slight, not overly) because of fluid needs. It is completely normal especially if you are EBF'ing.
the sunken\not sunken soft spot thing varies throughout the day! As long as it is not overly sunken it is perfectly fine, if it is overly sunken it is a sign of dehydration.
if you are concerned, gently pinch the nail beds of baby's toes and fingers - if they go back to their color quickly, they are hydrated well, if slowly, they are dehydrated. same with skin color as well (pinch gently) and if skin does not "bounce" back fast then it is also a sign of dehydration.
sheal (sorry, nak'ing here