Trackpad "zones" and selecting text?

Hi All,

When I want to select text, I click with my thumb, and drag with my forefinger. I've found though, that if I click (thumb) above about 20mm from the bottom of the track pad, I can't select text (in a browser it's like pressing the back button).

I've had my laptop for about nine months, so something has either changed, or I've mysteriously changed where I put my fingers. So I have two questions:

1. Is this normal behavior?
2. If not, has anyone heard of this happening before, and a resolution?

Ta, Banga