Something is using 20GB of space on my zenfone2(32GB) and I can't find out what it is

Recently I installed CM13.1 on my Zenfone 2. Since then something has taken up around 20GB of space, so I only have around 5GB of space to use. I keep looking for things to delete in my file manager, but their simply is nothing taking up that much space. In the storage settings page it shows 4GB of apps, 1.2 GB of photos, and everything else adds up to less than half a Gig. I do remember creating a backup of my old OS, (I think through TWRP, although I can't remember for sure.) Does anyone know what could be taking up that space? Or how I could find it and delete it?

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