It likely has to do with your first name, or a well known middle name. Ellie can be a diminutive of a much longer, older name, and is often used in place of those names.
Nicknames can have unusual origins, so do your best to remember when you first heard your nickname, and even ask relatives to help you find the root.
Of course, I noticed that you posted this in the dream section, and I was wondering if this is what you are called in your dreams and that is likely has nothing to do with your actual name. If this is correct, there are several theories that could explain it.
You have a connection to this name, either due to a family member, friend, someone you barely know who reminds you of something within yourself, or it is past-life recognition.
Which you choose is entirely dependent on what you believe.