Internal storage issue topic

Apologies if this could of been posted elsewhere but anyway.. I have a weird issue with my galaxy s4. I've been on stock, echoe ROM, and now cm12 and all have the same strange issue. According to disk usage a directory called system data is using over 3.5gb of internal storage. This wouldn't be a problem if it stayed that way, however every few hours my remaining 3gb internal that I have left after installing all my apps, goes down and down and eventually I get a "system storage low" message. Even overnight when the phone isn't even in use. Could anyone shed light as to what's eating my storage?

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