Your citrus is either suffering from sun damage or lack of nutrients. Citrus should actually be grown in full sun all day, so we don’t often see sun damage; however if the plant is in shade for part of the day, and then you get intense sun and heat (and/or dry soil or weather) suddenly, it can happen. Otherwise, we recommend keeping the soil evenly moist (only water when the top 2-3 inches of soil is dry) by watering until water drains from the bottom of the pot. Avoid fluctuations in very dry and then very wet soil. Once watering is done properly, if the plant still appears to have yellowing leaves, then you’ll want to fertilize with a material formulated for the needs of citrus trees. As your local garden center for an appropriate materials for use in container grown citrus.
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This plant has chlorosis – iron deficiency. Iron is necessary for producing chlorophyll and some enzyme functions. A plant will move iron from older leaves to newer growth which is why the yellowing leaves typically are seen lower down or toward the center of the plant in the early stages – blood meal is a great organic source of iron and will make pretty quick work of issues like this.