I must be seeing a different chart, because what I'm looking at doesn't even go to CD 25 yet.
On CD 18, you didn't really have what is known as Fertile Type CM (either watery or eggwhite) So I'm inclined to think CD 18 is not O day. Based
solely on temps, because that's all you have in there after CD 18, I would lean towards CD 20.
Did you stop monitoring your cm after CD 18 or did you just stop putting it into your chart? If you had your cm for the rest of the days on your chart, I would be able to give you a more definite opinion on what I think is going on because cm really corroborates whether O has happened or not.