This is usually due to a temporary glitch. It's happened to many of us several times, it isn't just you. It's probably only temporary, I wouldn't worry too much about it. If your rank does stay like that however, it's probably best to write a thread on the Halo Waypoint forums, as they're the official 343 forums (we're only a community fan-made forum). However, feel free to stick around, we love having new members on the site.