Nokia Nseries :: Phone Memory Low

Recently, i had removed all games and few applications , to make phone memory atlest 18 mb free. Then later i installed new applications on my Micro-sd card. everything is working fine. Then all of sudden after few days, My phone memory drops to 12Mb free. Why sudden consumption of phone memory and i tried removing internet cache and history saved. still my phone memory is at 12mb free. No new application is installed on phone memory. What could be the reason for this sudden and constant consumption of phone memory of almost 6mb