Sample.inddOK, I've found the problem.
Instead of real paragraph breaks, most of your breaks are just carriage returns, or soft breaks, followed by a tab which makes them LOOK like you have a new paragraph, when actually you have only five paragraphs in the entire document. Paragraph 5 begins on page 3 and runs to the end of the file.
To see this turn on non-printing characters (keyboard shortcut Ctrl + Alt + i). You also have lots of empty lines to create spaces between paragraphs, and these should be removed. Once you've replaced the soft breaks with real paragraphs your style definition includes space after, which is the correct way to make this separation.
As to WHY this happened, I have some guesses. Normally, the soft return is generated with the Shift + ENTER key combination, or using the Enter key on the number pad. Some computers, particularly some laptops with number pads, will swap the functions of the Num Enter and regular Enter keys depending on the state of the NumLock.
My guess, though, is it's more likely you've simply been using the Number pad enter key.