|Loss of leaves can be related to many different factors. These trees need very bright light and a relatively cool room temperature to thrive indoors during the winter. They also need high humidity when indoors during the heating season. A cooler room helps keep the humidity level up. Keep them away from both hot and cold drafts, too.
The soil should be allowed to dry out at the surface between thorough waterings, it should never be kept saturated or sopping wet. Over or underwatering can contribute to leaf loss.
Sometimes repotting can cause problems for the root system if for instance the new soil is not similar to the old in terms of drainage, or if the new container is much larger than the old one. (Citrus actually do well in a relatively small container.) Take care when you water that the old and new soil are both damp and do not let the pot sit in water in the drainage saucer after watering.
The sticky substance can be an indication of a pest problem such as a scale insect infestation or possibly aphids or mealybugs. If you suspect a pest problem, I would suggest you consult with your local county extension to obtain a specific identification of the pest and based on knowing that, recommendations for how to control it on an indoor plant. They may appreciate a sample of the affected foliage and a photo of the overall plant -- if it is not a pest issue they should be able to tell you that, too.
Finally, here is some general care information you may find helpful. You may need to cut and paste the complete url into your browser to make it work correctly.
I hope this helps you trouble shoot.