Music disappearing??

[samsungandroid12]

So I owned a S5 for about a year now. I have used almost all of my internal space. I love listen to music, so I have about 400 songs. During about my 5th month, I noticed when I want to listen to a certain music, I try to find it, but I noticed it has disappeared! I also checked my download file since I get all of my music from the internet, and it's gone, like I never downloaded it. It's not the first time this happened either, I had this problem for 7 months, and it's frustrating. It tends to delete about 2-5 of my music without me knowing. Does anyone know why it's causing this? Perhaps the excess amount of data I have causing this???