Hi hyejung0914,
It sounds like you're missing photos. We're glad you've joined us in Apple Support Communities. Let's see if this helps.
If you Restart your iPhone, does the issue remain? A restart is a great step when experiencing behaviors like this.
If the issue remains, follow the steps in If you're missing photos or videos in the Photos app. This link has the most frequent solutions to troubleshooting when you can't find your photos.
If the issue remains after following those steps and you've verified the iCloud Photos was on, let us know if you can see your photos when you login to iCloud.com. Also, let us know about how many days you've been receiving the iCloud storage warning.
We hope that helps with your question and take care!