I wouldn't rely on a URL shortening service. It's not very hard to do it yourself...
Yes, they will ask about your source. Either do as mentioned above, and have a site ready, or just tell them that they should know this business well enough to know that you never reveal a good traffic source.
Last time my AM called and asked, I just said, "Are the leads good?" and he said yes, then I said, half joking, half serious, "well, fuck you very much then!" He just laughed and said "ok, we're cool."