'Storage space is running low' while I have plenty of space remaining
This is really a common question from android users.

The answer is that Android has two type of space: app space and file space. When you install an app, it will use the App space (that is usually around a couple of gbs). When you do a factory reset, you will remove all installed apps (from app space) but file space will not be touched.

A common solution to this problem, is going to Settings -> Apps and try to move as much app as possible to the external sd storage.

Good luck