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The tree is adjusting to getting less sunlight indoors. While indoors, make sure you reduce the amount of water a little since it will have fewer leaves while avoiding the soil from getting dry. Water if the top soil feels dry. To get more sunlight, choose a southern facing location, consider using grow lights and increasing the inside humidity. Try not to expose the tree to temperatures below 45F. Over and under watering can also promote leaf drop. Drill more holes in a pot if you see that water is not draining well or consider using fresh new better draining soil.

It looks like sunburn. Here is more:

https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-leaves-turn-yellow.htm

It can take five or more years, if ever, to get fruit from a lemon tree grown from seed, and the lemons are typically inferior to the parent tree. If you really want lemons, it is best to buy a commercially grafted lemon tree which will fruit in two to three years and you know what you are getting.

https://www.gardeningknowhow.com/edible/fruits/lemons/propagating-lemon-seeds.htm

Unfortunately, your photos did not come through. I am unable to assess the damage.

It does sound like a severe infection, either bacterial or fungal. This happens, most often, when soil does not have a chance to dry out thoroughly down through the first few inches at the top.

Only cut off infected leaves. Do not cut stems. Cut all growth coming from below the graft point, as these will be a wild type citrus that may or may not fruit at all.

Treating with a fungicide will be necessary. This article will help:

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

This article will help you with the care of your Meyer Lemon:

https://www.gardeningknowhow.com/edible/fruits/lemons/growing-meyer-lemons.htm

Your photos didn't come through but your plant may have citrus scale. Here is information on control:

https://www.gardeningknowhow.com/edible/fruits/citrus/controlling-citrus-scale.htm

https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-not-fruiting.htm

Use a slow release fertilizer for your lemon tree with more nitrogen than phosphorous or potassium. Here is more:

https://www.gardeningknowhow.com/edible/fruits/lemons/growing-lemon-trees-containers.htm
https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=25682