Looks like maybe greasy spot; it is caused by a fungus that usually originates in dead leaves on the soil under the tree, hence orchard sanitation is very important. Greasy spot, like most fungal infections requires high humidity conditions. You can google citrus greasy spot and find lots of ways to treat it. I prefer to prevent it; and for that I use a systemic fungicide; and only stronger measures when there is a significant outbreak that has not been controlled by the systemic. I am a big fan of Bayer Silvacur; but there are other good products. Ask your local nurseryman.

i see in your pic that it had new growth and a good green color at the time you repotted. i'm thinking it has wet feet (bottom half of the mix being too wet) and/or not enough fertilizer. did you fertilize from the time of repotting until last week? citrus are fairly heavy feeders, especially with new growth and fruit production. it wouldn't hurt to fertilize lightly every month. and feed a little more when its actively growing/fruiting.

Well, clearly it has been sprayed with something that has caused some strange markings on the leaves. I would send your photos to Four Winds Growers and ask them what they think it might be. I suspect it will be fine, and will not affect the health of your tree. But, worth finding out what may have caused the discolorations and markings. Very odd, it appears to be a chemical burn of some sort. Patty S.

Looks like classic spider mite; you will need a 10X loupe to see them. Much advice on this site for spider mite treatment.