About a month ago I planted two lemon trees from seeds of a store bought lemon. They both grew and then one just started dropping leaves and I don't know what I did wrong. Any help would be appreciated.
Welcome to citrus growing by seed. My experience has a track record of 75% to 99% of all my batches of citrus seeds to die. To help you out You should get a less damp fast draining soil like 5-1-1 (5 parts pine bark fines, 1 part perlite, 1 part peat moss or equivalent. Put it in a well draining pot with the soil line flush with the pot brim so the top surface of soil dries. Water from underneath in a bucket but don't let the surface soil get wet. Remoce from the bucket and set on a draining surface so it cann't pull water back in.
Too wet of soil. Probably not enough sun. Seedling of any sort will tamp off if the soil/potting medium is too wet, especially citrus.
Just plant a lemon seed in at least a quart to half gallon sized pot with a very well draining mix and don't overwater.
Those cups are too small. The soil stays too wet or it dries out entirely.
You know, they don't look like they are at deaths door. Could it be that the dropped leaves are the spent cotyledons and the seedlings are doing fine?