Sunglasses lens are blurry?

I just got new sunglasses from Target. The first hour or so i had them on (i was wearing them for fun, and I forgot to take them off while walking around the house) they seemed fine, nothing wrong! But then I try them on again, and the right lens was blurry. I took a look at them under the light, and there were little lines on the glasses. I don't know the right word, but it looked like pleats in the plastic. The lines were raised from the plastic. I don't think its a flaw in teh plastic, because like I said, they were fine for the first hour! Can anyone explain what this is and/or know how to fix it? should I return them?