Text History from 1 phone # Disapeared from my "Messages" App

[AC Question]

I woke up 1 day, turn on my cell phone screen, & found my Samsung Galaxy S6 showing a Contact's Screen Page, for no reason. Then I noticed that the text messages that I sent to that Contact's cell number disappeared in my "Messages" App history. But texts to/from other cell phones remain in my "Messages" App history. What happened?! Why did this happen?!