Had you sent a bunch of other messages before this single one, though? (Like, had you send four different PMs to four different people, and then this was the fifth?)
If so, I could see it triggering some sort of flood message. I know I've gotten them when I didn't realize that a PM had already sent (sending it from my Blackberry) and then I tried to re-send, which read as me sending two PMs in quick succession.
Anyway, that's a guess. Probably best to use the "contact us" link to get a more definitive answer.