I'm sure you would have been in something other than a mw2 modded lobby to get a a suspension.
Check your email associated with your account and that should say what the suspension is for.

In all the times I have been in lobbies on mw2 I have never been suspended.

The short answer is no, if you say you joined the modded lobby, then that gives Microsoft a case. I'm afraid you'll just have to take this one on the chin. Watch out, if they see you've repeated your offense, expect a much more severe ban