Hello everyone, I am really hoping someone can help me with this and I will forever be appreciative

I was notified that my current tax refund was going to be reduced by $300 because I forgot to include a 1099 int form from the bank on my 2016 tax returns. I don't know much about taxes so I researched and found out I have to complete a 1040x for 2016 and can include the forgotten 1099 int form. I downloaded the forms online and obtained my 2016 returns as well as the 1099 int form from my bank but was a little confused on how to go about calculating the new tax on my income since my gross income increased by $300.

So not knowing what to do, I went onto freetaxusa.com which is the site I used to do my taxes in 2016 and found out I could submit a 1040x through them since I used them for my taxes in that year. While completing the 1040x and after including the 1099 int form from the bank which simply said I received $300 in interest income (which was a bonus I got from Chase bank for opening an account in 2016), the results showed that I OWED a little money for both federal and state. How does this make sense? I assumed that I would be refunded back $300 since thats the amount they are taking off my current 2018 tax refund. Am I missing something? I am so confused?