When entering that I contributed $5500 in my Roth IRA, my federal return estimate dropped by $500. Why is that?

If your refund decreased when you entered your Roth IRA contributions, that probably indicates that you have exceeded the contribution limit for Roth IRAs based on your income and the contribution is being taxed as an excess contribution. 

Go back through the section of your return where you entered the Roth contributions and be sure to go through each screen.  TurboTax will indicate if you have an excess contribution that needs to be removed. 

To read more, take a look at Roth IRA Contributions for 2017.