The interest rate and payment adjustments may or may not be scheduled to change at the same time. For example, the interest rate on some plans changes more frequently than the monthly payment, which may result in negative amortization. "This means that the additional interest will be added to the principal balance of the loan and may accrue additional interest itself," Hymer says. If the monthly payments on an ARM are increasing, generally this is because the index is rising or it is a negative amortization ARM.