Its probably your xbox. just turn it off and turn it back on and reload your save. its happened to me lots of times and one time lost an hours worth of gameplay

Your disk might be scratched, and this can affect extremely specific parts of the game. Heres an example. When I had my frist Fallout 3 disk, i put a large scratch in it. And the lady from the replicated man quest that comes up and tries to talk to you, that always froze my game. no matter what. And she follows you throughout the entire wasteland!!!!!!!!! I had to return the disk just to play it!!!!!! She wouldn't stop following me!!!!!!!!
as soon as I got a new disk and talked to her, I killed her >=D Now her decapitated body is permanently saved in the waster around the Megaton bomb. Anywasy, this could have happened to you.

This is a known bug with no known cure, all you can do is make sure you have the latest patch installed (1.5.0.23 for the Xbox and 1.6.0.3 for the PC) and start a new game and hope it doesn't happen again. You can also try deleting BS and re-downloading it.

It's a glitch. The only way to get rid of it is to delete your saved and autosaved files and start all over

A scratch on your disc wouldn't have anything to do with it, lvl 21 is DLC not software. I'd go with itwizz' suggestion of re-downloading Broken Steel.

No @chrisyg103 is right when ur disk is scratched certain parts of games can crash its like when u bang ur xbox lightly ther are so sensetive that if it even wobbles or gets banged slightly u look to ur right or left and al u can see is grey so its a silly glitch