Actually it still could be memory. Your card is seperate from the device memory.
How much memory do you have? If it's less than 16mg you might want to review: Why is my BlackBerry losing its call logs or message logs? - BlackBerryFAQ
Another culprit is not closing applications correctly. If you use the Red End key to exit an application it will leave it in memory.
In any case if you never have issues then I would say a battery pull should
clear things up!