IPhone 5s back glass cracked?

I don't know if my glass is cracked or scratched. In the bottom glass in the center there appears to be a hairline crack or scratch. I have had the phone for 2 months and I have never dropped it and I have had it in a case since the day I got it and I just noticed the crack today. I think it is a crack because it looks like their are 2 tiny pieces of glass missing and when I run my fingernail over it I can feel it catch. Is my glass defective because I have never dropped it and I don't want to pay $150 to have it fixed what should I do?!

In fact, you can only replace the bottom back glass, which is very cheap. The cost of the bottom back glass is only about less than $5. But special tools and skills are required. You'd better ask help from an expert.