Font size changed?

I have had this happen a few times. I typed a page and looked back at it. One half of one sentence switched from 12 pt to 11 pt. I am not sure how this is happening, am I hitting a key inadvertently? Thanks for your help.

Probably… 8)

⌘- (hyphen) will decrease the font size, even while typing.

I guess I must be hitting that combo then, thank you!