Welcome, hope we can help out. Gosh, that little lemon sure does look like it's in trouble.
First, sorry but I have questions - where are you located? You can put this into your personal profile so it will always come up in your posts - see mine in the upper right corner - just your city and state will be helpful.
If you're in Florida it's more than likely your lemon is infected with Citrus Greening disease. Not much you can do for it if that's the case. Looks like the whole plant is involved. Extension services are not recommending anyone plant citrus in Florida until the disease is under control.
More questions: how long have you had it? What are you fertilizing with and how often? What soil did you use in the pot? How much water do you give it, and have you checked the pH of your water? If you're using untreated well water, for example, it could be too high for use on citrus.
