Why am I a recruit again?

There is two(-ish) potential reasons for this:

1. "I just started with Xbox live"

   That very well could be your answer. In Reach, offline credits are calculated differently than online credits. When you go online for the first time, it converts your offline credits to online ones, and it takes a lot more online credits to get to General than it does offline. Now, you should be a lot higher than a recruit, but it may be that it is in the process of figuring out what your rank will be (or it just reset it and 343 Industries has a different policy than Bungie, but I wasn't able to find any evidence on this).

2. Another reason that credits get reset is if it detects that you are trying to manipulate things. If you are using glitches to do things such as reset challenges, your credits and armor can get reset by the Banhammer. Sometimes this will also include a temporary ban, especially from earning credits, but not always from what I've heard.

Both of these systems are in place to prevent exploitation of the system so that users have to earn their armor and rank, instead of just glitching to the top (as has been an issue in previous Halo games).