Why did my flash drive run out of memory when I deleted everything?

I recently got a new macbook pro, and to transfer my itunes library I bought a new sandisk 12 gb flash drive. I put about 7.5 gigs of music on the flash drive and put it on my new computer, and everything worked, so I deleted all of the songs from my flash drive. When I put the flash drive back in my old computer to get the rest of the music, it says I only have 244.7 mb left. The only things I did not delete from the flash drive are autorun.inf, RunClubSanDisk.exe, and a folder called club_application. If anyone could tell me what happened, and how I can make it work again, it would be greatly appreciated. Thanks a lot! :)