Memory fluctuates constantly

Hi, I have a 5c that is about a year and half old. For some time now my available memory goes up and down. I'm not talking about a couple of MB's, I'm talking about hundreds of MB's. iTunes says I have 1.2 GB free. The phone says I have 350MB. Then, if look later in the day, without doing any downloads, taking any photos or doing anything that should significantly take up memory, I'll be down to 100MG. Then the next day I'll be back up to 400MB. Sometime I even get a random message saying my memory is full and I need to clear space. I'll check and I will indeed be down to 0MB free. Since I have been typing this I have literally lost 10MB and my phone has been sitting completely idle. I understand that auto updates files will take up some space and then delete themselves, but why the huge swing. These updates files are typically not that large. Certainly not 300MB worth. Also, why does iTunes show I have so much more space than the phone does?

Thanks!