Why is my storage full?

My phone keeps saying my storage is full, but when I check my settings, I have 4 GBs left. Now I can't text,call, or search the web on my phone without it closing because I have no storage. If I restart my phone, I get 50 MBs that last me about 3 minutes before I have to reboot it again. Also, my alarms, notes, reminders, and calenders are getting deleted for some reason. My phone also freezes frequently and is very slow. This hasn't happened before. I've had to delete 80% of my apps, but I still have no space. My photos have the most space, but I can't delete some of my photos because the app keeps closing on me.