Why does my Samsung Galaxy S4 tell me the internal storage is full? I've installed just 3 apps!!?

[Olgha]

Hi People,
Need some your help pls. Apart from the normal apps that come with the samsung galaxy s4, i added just Facebook, Twitter and WhatsApp. I've added no videos, audios or even pics. Yet as soon as i opened facebook today (that was the last app i added) it immediately started telling me "low disk space, storage is full."
Could someone pls tell me what to do about this?. It's supposed to have 16gb of space so i really dunno what to do. I dont even know what to delete coz ive barely put anything on the phone.
Could someone help pls??

[Vivek]

I thing you may have fake S4 look in settings>stoage and reply your storage info if its above 8gb its genuine if not you might have a situation