Citrus Issue

This might be the result of a watering or temperature issue or it might be the beginning of some sort of disease. It is hard to diagnose disease with certainty without lab testing. It does not appear to be a nutrient issue so hold off on fertilizer until you get the other possibilities figured out.
You don’t say if you planted it in the ground or in a pot or when it was purchased.
If you purchased it last week, I would immediately notify the place where you bought it, take this leaf and request their assistance. A photo of the whole plant would be helpful also.
If you planted it in the ground, do you know what type of soil that you have? Does it drain quickly or does water just sit? Citrus trees require well draining soil. That is easier to control if it is in a pot.