Hi K.,
I read the responses to your question and although many responders touched upon the potential explanations I would like to add that babies around the age of your child begin to develop memory. When this period occures we call it "Object Permanence". During Object Permanence children realize for the first time that they know mommy and daddy and other regular caregivers, but they now know that people look and sound different than those with whom they are most familiar. This makes children anxious and fearful. Behaviors associated with this developmental period include, baby crying when you walk out of sight, becoming fearful of others and taking a bit of time to warm up to them before they will go to another person, clinging to you when others are around, and waking in the night and crying out to make sure you are there.
Before this period of brain development, baby would go to anyone and would seek the attention of others to play and socially engage. Before this period, baby slept through the night.
It's confusing because when baby woke in the night during the time they needed fed every 2-4 hours it was a signal they were hungry, right? Not now. Now, it's because the baby needs reassurance that you are there, that's all....of course, unless it's teething or illness.
So, the recommendation is to reassure your baby when you first hear a sound telling you baby is waking up. Begin by softly telling baby you are there and it's ok, go back to sleep or shhhhing. If that isn't enough move to placing your hand on the baby or doing something you know that soothes baby without picking baby up. If nothing but picking baby up works, try to hold baby as little as possible to comfort baby. The idea is to not get sleep off track if you can possibly do that. Feeding baby isn't recommended.
This period can be mild to severe, depending on baby and that can all be normal. This period ends between 12-15 months for most babies and sleep will be back on track once again!
Hope this helps explain this developmental stage as it relates to a child's behavior and that it is helpful information for all of the wonderful mommies!
You're all doing such a wonderful job!