Problem with my Mac

I am a graphic designer and recently was working on a project with a huge file size. Out of nowhere my computer said "out of nowhere your mac os x start up disk has no more space available for aplication memory" and I had to force quit tons of programs.

I reset my computer and left it off for a few minutes then only had 1 program open and I got the same message.

I have tons of hard drive space, i have no idea whats going on. I have to meet a client in the morning and it is not looking to good?

ANy ideas?