The same issue started with my iPhone 6 back in January. It would go from having nearly a full charge to dying. Once the phone was plugged back in to power, it would boot up and show 90+% battery. It didn't happen everyday, but it got to the point where it was happening once or twice a week and if I didn't have a power source to plug the phone into, I was hosed.
As the other poster said, it's likely happening because the battery is worn out. You can get the battery replaced by Apple for around $80 USD, I believe. In my case, I decided to trade the iPhone 6 in and I bought an iPhone 7.
its normal for an iPhone 6 considering its a few years old now, happened to every iPhone I owned after around 2 years of usage (4, 5, 6) then I would just trade in for a new phone. it kinda sucks though if it wasnt for the worn out battery i might still be using my iPhone 6 now