New iPhone photos lost

I bought a new iPhone 6S earlier today and started taking some photos right after activating it. I didn't connect to the iTunes or cloud at the time of taking photos. Now I got my connections setup and all the stuff from my earlier iPhone got synced perfectly. But now I have lost all the new pictures that I took today. I think the reason is because of the backup that overwrote everything. How can I recover my lost photos?

Thanks and appreciate your help.