Certified GKH Gardening Expert
In container, these can be difficult to keep unless you have a few things handy. You will want to always have, on hand, dolomitic lime, and iron sulfate. Mix these in twice a year, and feed a good granular fertilizer once per year.
The most likely culprits would be, lack of potassium, and phosphorus, OR a severe pH swing... This would be the more likely reason.
Both dolomitic lime, and iron sulfate will keep the pH where it should be, and provide magnesium and calcium. These two will always be lacking in container citrus.
This article will give you more information: https://www.gardeningknowhow.com/edible/fruits/lemons/growing-lemon-trees-containers.htm