that's odd, first make sure that it's space used that you're looking at.
if you use windows, do pc maintenance with ccleaner's registry cleaner, windows' disk defragmenter (not for ssd), and windows' disk cleanup
if you have an antivirus program, do a full scan. if not, download microsoft security essentials, update it, and run a full scan. full scans take longer, but they're more thorough