When you have time machine enabled it will backup to your internal and move those files to your external time machine the next time its plugged in.
I had that once (92.3GB of backups) but it only showed up in the storage section of "about this mac" my actual disk space wasn't any corresponding with that info (About this mac showed 240.6GB free, actual free space was 332.9GB) right click your Macintosh HD icon and click get Info. there it will tell you the actual free space. Don't worry about what it says in that storage section. Plug in the external and it will be gone. And you get the apparently used up space back!
For all those who don't know what the "other" section is. Its all your Documents, System files, downloads, etc. Everything else on your computer that doesn't fit into one of the categories: Audio, Movies, Photos, Apps.
Download Disk Inventory X to find out the location of all files that hog your disk space. I found approx. 180GB of stuff that were transferred from old macs, I thought those files didn't even exist anymore.