It's a strange thing that has happened to your USB. Have you tried to scan it against viruses? If not, I think you should do so. Besides the antivirus and antimalware software, you can also check the USB drive for errors.
Open the Command prompt and type the following command:
chkdsk X: /r
Then press Enter. One important thing! You must replace the letter X with the one which has been given to your pen drive by the PC.
Let me know if that helped. Cheers.