Centered text that does not stay centered

[SteveA]

I'm putting together a collection of sayings, which I'm arranging numerically one after the other, with the number of each saying centered above it; the saying itself is aligned left. Once in a while, all the centered numbers in the text suddenly shift to the left margin, as does the centered title of the work itself. At the same time the font size of the title changes from 14-point to 11-point, which is the font size of the basic text. Could it be that I'm accidentally hitting a key or a combination of keys to cause this? If so, how do I reverse it?