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I would suspect that the lemon has been treated with a pesticide or another chemical that is not suitable to be eaten. The other possibility is that the fruit quality is so poor, that the fruit will not taste that good. Some citrus trees are meant to be grown for looks rather than as a food product.

As far as I know, there are no lemon trees that are dangerous/poisonous to eat the fruit from, so I believe it is the care or quality of the fruit that makes it not suitable for comsumption.

Sometimes too many lemons on one plant can cause them to grow
smaller, so be sure to pick them regularly. This article should help with growing lemon trees: https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm

If it is only 5 months old, the tree is probably purging the buds because it is still too small to support the fruit the buds would produce. This is normal. In fact, I would recommend that until the tree is 2 years old, that you remove any blossoms it produces so that it can focus on becoming well established rather than on growing fruit. It will be better for the long term health of the tree.

It is normal for limes (and occasionally lemons) to drop their "pea size" fruits just after flowering in spring. They may also drop them again in summer due to the warmer temps. Excessive dropping could be attributed to stress, however.

Here is an article that you may find helpful: https://www.gardeningknowhow.com/edible/fruits/lime/is-lime-fruit-and-lime-blossoms-falling-off-tree-normal.htm

If the leaves are green veined, but yellow else where, the plant has a iron deficiency. This article will help fix that:
https://www.gardeningknowhow.com/plant-problems/environmental/leaf-chlorosis-and-iron.htm

This article should help with growing lemons: https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm

Growing lemon trees from seed is similar to that of lime trees. Therefore, this article should help: https://www.gardeningknowhow.com/edible/fruits/lime/growing-lime-trees-from-seed.htm