What size hard drive do you have in your MBP? I recommend having a minimum 10% free on your hard drive. Mac OIS X needs room to breath and with only 3GB, it doesn't have much room. That 3GB of space will become fragmented fast! I would say you need to open up some space on your hard drive.
For why it now says 0KB, I agree with the first poster, probably virtual memory or temporary files etc.