Trouble Typing

Frequintly while typing here or elseware, about now... the screen suddenly scrolls down and sometimes to somewere else.
I have been two or three lines into a response, and suddenly I'm somewere else. Then I have to find my way back to were I was and start over.
Is my thumb hitting the laptop mouse pad thing, or is this something others have had happen.
I can work around it, but I'd like to know if it's me or if it's a common problem I can fix.