My lo is 18 weeks old, but she is still smallish for her age, weighs 12lb 5oz, she normally has 4 feeds a day of 6oz the very odd occasion 7oz but for the last two days shes been screaming for a feed like shes really hungry but only taking 3oz, i am worried shes not having enough milk becuase shes only having 4 3oz feeds. What should i do??