One option is to italicize the word okay:
This happened after she told me okay.
This type of italicization is known as "using the word as a word."
If you followed British punctuation conventions, it would be clear from the outset whether the period was included in the original quoted expression, because British conventions call for putting the end-of-...
When you want to emphasize that someone used an explicit period, you can use the word "period".
This happened after she told me "Okay" – period.
This also expresses the abrupt implication. From Merriam-Webster:
used interjectionally to emphasize the finality of the preceding statement