cabbie - 10:11 pm on Feb 24, 2013 (gmt 0)

To me, your situation is describing what I think Google would do, if they suspected invalid clicking.
They show revenue in analytics and then take it away.

If you traffic seems to be exactly the same for a long while now,and assuming that it was always valid traffic, then one possible scenario, I can think of, is that a hacker has hijacked your traffic, perhaps redirecting to their own site. But, so as you do not get alerted,they create dummy traffic for you, even to the point of having an average CTR for your displayed ads.
A site making $100 a day could be an attractive target for such a scheme.
You may need outsourced testing from different locations and browsers and check different serps for your traffic and make sure there is no hijacking going on.