System takes up 83 GB of space

Storage on my MacBook Air 11 2011 fulled up, so I moved all the music from iTunes Media/Music to an external hard drive and then deleted it from the internal 128GB SSD. After finishing work with the files I needed the space for, I tried to move all my music back (about 50GB), but it happened, that i had only 30GB left, and I can't indeed move it back. Storage report @ About This Mac | Storage says that system takes up 83GB of space, which before was around 30GB. OmniDiskSweeper on the other hand sees only about 30 GB of taken up space, which is more like true. All the measures to find out what exactly takes up so much space have failed, and to note all the files and apps that I worked with after deleting the music I have also deleted and now rebooted the MacBook a couple of times, things don't change. And, sadly, Dr. Cleaner is not available to download from Mac App Store in my country (Russia). Any possible help from you guys? I work as a DJ, as you probably can tell from my nickname, and I sort of need my music back.