My MacBook Pro HDD says it's filled with backups

Hi, I got a mid 2012, 13'' MacBook Pro, with the 2.5 GHz Intel Core i5 and a 500 GB HDD, running OS X Yosemite 10.10.3

Recently when I was installing an Microsoft Office update my computer said it was filled with backups (in Danish Sik.kopier), I had no trouble whatsoever installing the update. When I afterwards checked in "About this Mac" it said that 140.75 TB were filled with backups and I still had 339.53 GB available (in Danish tilgængelig).

I haven't yet experienced any problems but I wondered if any of you have seen this before and can explain me why???

Thanks in advance