I started using this skin serum made to lighten my skin. I only wanted it to lighten a certain area and now it made my skin even darker. Why is that and what can I use that will work in the least amount of time?

I guess you used a product containing Hydroquinone. Sometimes darkening can reoccur ofter you stop using the product. The solution is to use products containing Kojic acid vitamin c etc. Also you must never go in the sun without a high level of sun protection.