I have dark spots and splitting of the lemons on my tree what do you think?

Question

I live in Mexico in the mountains; it is the rainy season here.

Answer 1

This is a fungus that's invaded your lemon tree. A visit to a local nursery seems like your best option. I hope these articles will help you to understand what has happened.
https://www.gardeningknowhow.com/plant-problems/disease/alternaria-leaf-spot.htm
https://www.gardeningknowhow.com/edible/fruits/citrus/citrus-alternaria-rot.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/common-lemon-tree-diseases.htm