Revenue Officers in the Collections Division of the IRS often go to great lengths to determine the financial situation of people who owe back taxes. They are smarter than many taxpayers would think. If you are following your installment agreement, you should not worry.
This is not intended to be legal advice, and is general in nature.
Did the IRS personnel tell you they checked your credit report one month before you filed your taxes? How did you find out the IRS checked the report? It seems odd they would take any action as long as you have remained current and compliant on your payment agreement. Once you enter a payment agreement the IRS generally does not have the power to continue additional collections measures until your period for your current agreement ends (typically two years).
This answer is based on general legal principles only and is not intended to provide specific legal advice. This answer is for informational purposes only and does not constitute the formation of a lawyer-client relationship. Any reader of this answer should not make decisions based upon in without first directly consulting with an attorney Circular 230 Disclaimer - “Nothing in this response is intended or written to be used, and cannot be used by any person for the purpose of avoiding tax penalties regarding any transactions or matters addressed herein. You should always seek advice from independent tax advisers regarding the same.
This, indeed, is quite odd - and apart from the question of how you found out this information (probably by requesting a credit report of yourself) - I think it is safe to imagine that if you owed me money (could be a big chunk of change) i would be interested in your ability to pay me now and in the future hence a credit check. I wouldn't put anything past the IRS especially if you have back taxes. If this persists or affects you in anyway, contact a tax attorney!
This information is not to be construed as legal advice nor does it establish an attorney-client relationship. It is only my opinion based on my expertise, the best advice to follow is that derived from an attorney-client relationship, not a web-based response. Good luck!