Here is an update to my post and a response specifically to yours because we had a similar issue.
So luckily I had an appointment with apple ahead of time for the discoloration of the screen. In all honesty the hairline crack didn't bother me too much and I brought it to their attention right away. I told them that I did not care about the crack as much as I do about the discoloration. She informed me that ONE crack on the screen is covered under warranty(Made me think it is a standard issue on the i6 and i6+ to receive a single crack). I handed the phone over to her and she ran standard tests and checked for water damage. Everything checked out and she said they will replace my screen and digitizer while I wait. Of course, I agreed to that and she took my phone to the back. Not even a minute later she comes to me and tells me they cannot repair the screen. Why? Well, she laid the phone face down on the table and it would waddle from corner to corner. The phone literally warped itself. I don't wear skinny jeans, I don't keep my phone in my back pocket, as a matter of fact I rarely keep my phone in my pocket much at all. I'm probably on my phone 90% of the day because of business needs. But anyways, she said she seen a few cases where the phones metal expands and contracts because of turning hot then cold eventually what happens is either the screen pops off because it doesn't fit in the warped housing or the screen cracks. Well, in my case both things happened. End of it all, I walked out of apple with a new phone. Or, newly refurbished phone. So that was my situation.
TL:DR Screen cracked by itself. Took it to apple. They replaced my phone.