If "private" browsing on iPhone is anything like 'incognito mode' on Google Chrome, then yep, that will do it!

The browser generally uses cookies or some other method to track what you've done (thereby controlling the text turning yellow on a post you've read). "Private" browsing modes may or may not allow those cookies/trackers to be stored... if they do allow them to be stored, they automatically wipe it all out when you close the browser. Therefore, next time you open the browser, it's like you're starting completely over...

Glad you got it sorted!