Enforcing the contract you signed is not illegal and does not constitute illegal retaliation. If you are actually behind on your rent, the landlord is generally not required to give you time to catch up. You could attempt to argue that the landlord waived their right to timely rent by entering into an agreement with you, but unless you provided additional consideration for the agreement (such as agreeing to pay extra in exchange for the additional time) then the agreement is likely not binding.
You can reach Harkess & Salter LLC at (303) 531-5380 or info@Harkess-Salter.com. Stephen Harkess is an attorney licensed in the state and federal courts of Colorado. This answer is for general information only and does not create an attorney client relationship between Stephen Harkess or Harkess & Salter LLC and any person. You should schedule a consultation with an attorney to discuss the specifics of your legal issues.