I do not dye my hair but lately I have seen a pinkish highlight on the ends. I am using the same shampoo, conditioner, and protectant as I have for a long while. The shampoo is clear. I do use a flat iron on it every day. Could that be the cause?

Perhaps this may help
https://www.simplyhair.co.uk/causes-blonde-hair-turn-pinkorange/

Our basic natural hair colours are made up of many hues, but changes over time as we age. You probably have a basic ash colour and that is why the pink colour. Nothing you are doing wrong, but you could probably have your ends trimmed every 4 months as the ends are more prone to absorb chemicals from the products you are using on your hair.