So are you promted with this message upon logging in? Are you still able to acces files/ and or use your user/ account?
A alot fo the time your mac will lie to you about your start up dic being full... Start here:
upper right hand corner>about this mac>more info>storeage
what does that bar indicate? what is filling up your computer. If you are seeing larger ammounts of other (ie 50 percent or greater) then it is likely user specific. If it is i would bet a fair ammount of money that i might be realted to your email... check mac mail to see if you ahve snet a lerge attachment or if you have recieved one recently. If so i reccomend deleting it as it may back up your computer.
IF about this mac shows larger ammount of video or photos.. well its pretty self explainitory. Youwill either ahve to hack them up onto an external HD or remove them.
Hope this helps a little bit!