my two cents:

There are different types of Interest Only loans and the rates can be adjusted annually or be fixed for a period of time like 5 - 10 years. During this period of time you only need to pay interest for your mortgage and the principle will remain the same. So your monthly payment really depends on the interest rate that you got from your lender.

It seems like that your loan is Payment-option ARM with minimum monthly payment instead of IO.

I think you might want to talk with your loan agent about this.

Hope this helps.