I bought a small lemon and kumquat bush 2 weeks ago (image 1+2). They come originally from Sicilly. I put them on the balcony. The lemon was repoted (normal soil, not a special citrus one). It was sunny and warm the whole time now in may and i think the night temperatures didn't fall under 10°C. I fertilized them with a normal liquid fertilizer only once, but even before that, (during these 2 weeks) something started to happen (image 3+4). The lemon has brown spots on its leaves and the kumquat leaves look like they are withering. I watered them regularly, almost every day, because it was very warm outside. I also have a lemon tree (image 5) grown from seed 7-10 years ago, but it hasn't had flowers yet. I've noticed that it has brighter leaves than those (grafted) sold in the supermarkets. Sorry for my bad english.
Withering Citrus Plants
Certified GKH Gardening Expert
It does look like it could be sun damage. Avoid splashing water onto the leaves. Water at the soil level. Make sure the soil is well draining and that it is not holding water.
Check the moisture levels before watering.
Soggy soil can cause root rot and fungus issues.
These articles will help you review care.