iPhone 6 circular blue screen stain

I've searched on here and cant find a very similar issue, so was wondering what might have caused a blue circular stain to appear on my IP6 screen just above the home button and slightly to the right. Never been dropped. Best Buy would not do a battery replacement bc they seem to thinkit could be a swollen battery buy there's nothing visible. Nearest Apple store is 1.5 hours away and out of warranty etc anyway. Any help would be greatly appreciated. Thanks.