music disappearing from phone

Music that I have purchased from iTunes directly from my 5S has disappeared from the phone music library. I'm not sure when exactly it started, but they are disappearing at random. I never connect my phone to a computer for syncing, only to charge (connect to my work computer to charge, but it is not linked to an iTunes account on this computer). The music does not all delete at once, but a little at a time. At first I thought I was crazy and was thinking of music that I had downloaded to my iPod and not my iPhone, but now I'm certain that songs that have been purchased & downloaded directly to the phone are no longer there. They are in my cloud to re-download, but I need to know why they are randomly disappearing one by one from my phone. It's been old and new purchases that are disappearing. I'm still operating on OS 8.1.2 and have not yet downloaded 8.1.3.