While not completely uncommon, drops in rankings are a little rare. If you have only added to your page and increased word count and increased important term usage without changing anything else on your page, you will usually only experience increases in rank. Drops usually occur when something has been removed from a page. It is a good idea to double check that you didn't inadvertently remove anything (and we mean anything at all) from your page.
Here's what you should do:
1.) Make sure you know exactly all the changes that were made. If you are unsure, go back and through your website's editor history and make a note of all changes. It is critically important to know exactly what was changed on the page. You will be doing SEO in the dark and could harm things further if you don't know what you have done to the page.
2.) Give things a little bit of time.
2a.) If you did any decreasing or removing of content from your page and you haven't had a recovery in 5-10 days from the last cache date*, revert the changes back.
2b.) If you only did increases to the page and absolutely did not remove/change anything else and you haven't had a recovery in 14-21 days from the last cache date*, revert the changes back.
3.) If you revert/you get your rankings back. For the next round of tweaks, try only doing a couple of things at one time and see how the page responds. It is possible that for your niche, or this particular page, that you’ll need to make your changes in very small batches and the changes spaced out with more time than usual.
*A cache date is the date that Google crawled your page and took a picture of it. To see the cache date of your page put cache: in front of your URL. For example, if your target page was https://yourdomain.com/your-page you would put in: cache:https://yourdomain.com/your-page and click to search in Google. Google will show you the picture it has taken of your page and the date the picture was taken. Cache dates are good for confirming that Google has crawled your page and has seen the changes you have made to your page. If you see that your current cache date is prior to any changes that you have made, it means that it is unlikely that the drop you are experiencing is due to any of the on page changes you made.