New songs added to playlist disappear every time I sync my iPhone with iTunes

About a week or so ago, I bought two albums. They downloaded onto my iTunes and into my phone without any problems, and I was able to add them to one of my personal playlists without any problems. However, every time I plug my iPhone 5C into my Windows computer to sync/charge with iTunes, the songs keep getting erased from my playlist and I have to manually re-add them again. I'm not sure why this is happening. Has this happened to anyone else? How can I fix this?

Any help would be greatly appreciated. Thank you.