Do you have a table ending at the bottom of page 20? Tables have obligatory paragraph marks after them. You can resize the final paragraph mark to 2 pts with zero points above and below, which may do the trick, or by shaving space on the page, if possible. To see the hidden paragraph marks, press Ctrl+Shift+8 (the one above the "U"). This combination toggles hidden characters on and off.

But it would help to know exactly what you've got in your document.