By in use do you mean that amount of memory is being taken with stored data, or that amount is available? I initially thought you meant taken, but as I read further I'm not sure.
Assuming you have the free memory, I would try resetting the device. See KB02141-Reset a BlackBerry smartphone
Try a soft reset first. If necessary, try a hard reset.
If a hard reset doesn't correct the issue, look at possible app conflicts and system resources. If there is anything you can think of that you did prior to this issue occurring, like installing an app, undo it, such as deleting the app, and then test.
You might do a full backup and then use Link to reset the device to factory defaults. An alternative is to do a security wipe instead. Afterwards, test first, then restore step by step while testing between steps. The culprit could be an app, could be corrupt settings, and may turn up again as you restore and install apps, or not. If you are testing and paying attention you may catch it when and if it happens.