No longer whitelisted?

I'd been approved about a month ago for API whitelisted for my application. I'm not sending a huge amount of replies, (actually, I'm not even sending tweets, just pulling a few from user's timelines). The reason for whitelisting was because our service needs to auto-follow its users, so it needs to be authed.

Anyway, it's worked great for the past month. Yesterday, however, we've started to get errors like so:

<error>
Rate limit exceeded. Clients may not make more than 100 requests per hour.
</error>

And they've continued into today. According to Twitter Status the API is green, so what could be the problem here?