How many apps do you have installed on your main memory? Make sure you are saving your mp3's ..videos and other big files to the storage card. Obviously you have something else that has taken up all your memory.
Don't bother with the built in task manager. It sucks. Instead go here ---->
http://www.dotfred.net/TaskMgr.htm and download FDC Task Manager. IMO the BEST TM app for WM. Does reg editing also.