The only thing my mom could think of was jealousy. As in, she's jealous that we're talking and not including her. It's as plausible as anything I can come up with, but why did she wait until 10-11 yrs. to start this behavior? I am seriously considering taking her to the psychologist who diagnosed her, and demanding he fix it! (Oh, OK, I wouldn't put it quite that way, but that would be the gist of it.)
Since she can't answer the question "Why are you unhappy?" I have to assume it's an emotional issue she just doesn't understand. If the conversation continues, it will escalate to angry sobbing and screaming, and that's even more annoying than the whining. I really (really!) want this to stop.