Well, that babies don't necessarily grow continuously over time... They have growth spurts - well my DS did anyway. It helps to be aware of this and recognise the signs. I learned how to be in tune with DS's growth spurts as he would become irritable and his sleep pattern more erratic during these periods. He would teeth during this time too. It helps to know its a spurt because you can be reassured its temporary and settle down in a few days.
When babies suddenly swing out of their routine, it's quite stressful for new mums - don't worry, it may just be a growth spurt and will calm down. I wish I knew this, but did start to recognise it after a while.