Unjustifiable installment increases

Currently my minimum monthly installment is increasing each month without myself making any purchases. I asked the TFG representative to provide me a reasons and she was very rude and could not provide me with a reason. I have not missed any payments and therefore, my installement should not be increasing as TFG is threathening me as the are expressing that if I do not pay the increased amount my credit score will be negatively affected.