How common is IUD pregnancies?

I got my Skyla IUD inserted back in may after the birth of my son in march. I haven't had any issues with the IUD since it was inserted and I waited an entire month before having sex and since then my husband and I have gone raw every time. Well lately I've been feeling ...(eh)??? I've been getting headaches that are more frequent but are mild, my ribs and chest hurt a lot of the morning but go away by mid afternoon, I'm always hungry or wanting to snack, and I am always tired for some reason. I took a few test that showed negative. Should I be concerned or worried?