Retired in 2015 how do I request past LES and all Record of Reenlistments, etc…

I received a notice from DFAS stating that I owe $20,000 for a reenlistment bonus supposedly given to me in 2008. However, I never received this bonus, nor did I reenlist in 2008, as I was already on an indefinite enlistment that began in 2006. Additionally, I did not reenlist for any bonuses after my initial enlistment in the reserves, where I did receive a $5,000 bonus. I was given $2,500 of that bonus and had to repay a prorated amount when I changed my Military Occupational Specialty (MOS).

I need to request all documentation to prove that I never reenlisted for the bonus and that I never received any payments related to it. What other steps should I take?