recently purchased songs are being deleted

I have the iPhone 5. A couple weeks ago I was buying new songs in the iTunes app and just a few hours later they were all deleted. I redownloaded them through Purchased Music in the iTunes app on my phone, this process repeats a few times a day and happens with all my new songs. I'm not deleting them and I have them checked off for when i sync on my macbook. Why is this happening and how do I stop it?