All good thoughts there, greene. The leaves do look a bit dehydrated, as normally they will be shiny. These trees (citrus) really like humidity, so if you recently moved it from outside (more humid) to indoors (dry, with heat?) it might be responding to the sudden change to "dry weather". Keep a spray bottle of water handy, and give it a good spritz whenever you pass by?
I also think there might be some insect activity that would account for the rusty spots along the veins of the leaves.
Andreia, could you take a hand lens (magnifying glass) and look closely at those rusty spots? Also look carefully at the undersides of the leaves as well. If they are bumps, try scraping some off with your fingernail, and see if they might be scale insects. If they are, then a spray of horticultural oil, or the home-made substitute, a couple of teaspoons (40ml.) of cooking oil, plus a few drops of dish soap in a liter of water (shake well to keep the oil in suspension while you are using it) will help. Spray the whole plant well, wait maybe 5 days and spray it again. Be sure to hit under sides of the leaves as well.