Rarely, but MEMORY FILLS UP.

I have a problem which is rare but IT ANNOYS YOU AS HELL. MEMORY, OF ALL SUDDEN, FILLS UP. I HAVEN'T UPDATE MY ANDROID, I HAVEN'T DOWNLOAD ANY APPS WHICH CAN BE HEAVY (1-2GB ). MY SM-T320 WAS WITH 4GB MEMORY FREE. THEN IT SUDDENLY FILLED UP MY MEMORY TODAY, AND LEFT ME WITH 500MB. I CLEARED SOME MEMORY TO 2GB. BUT STILL, WHAT A HELL?!