Startup Disk is full??? macbook?

I have a MacBook Pro and whenever i upload something, a pop-up consistently comes up saying that my "Startup Disk is Full" so when i check the info page, it says i have 2GB left but when i add up all my files and my family member's files, we still have 40 GB left?!? completely weird.. any suggestions on what is happening? Thanks, much appreciated! :)