probably some bad data got in the upload file. If you have that file still on your device or on your computer, post it in the forum for your device and maybe someone will clean it up for you. Or if you are concerned with your privacy you can get the .fitfilerepair tool for a small fee and fix it yourself. If your device doesn't use .fit files, then you likely can manually edit them yourself as they are likely .gpx or .tcx files, which only need a text editor. But you do have to have some understanding of mark-up type coding.
Some of the other sites do more filtering and massaging of bad data in the upload file, so you might even try uploading to them, and if it looks better, export it from there back to your computer and import it to GC.
