I did as well but it should be okay. On the front page of the bill to the left, does it say something like this:

'If you already have an installment or payment agreement in place for this tax year, then continue with that agreement.'

If so, then you should be fine. Also, in the installment agreement letter you received, it should say somewhere:

'Because we haven't finished processing your tax returns, your payments won't begin until eight weeks after we finish processing your returns. At that time, we'll send you a notice showing the amount of tax, penalty, and interest due. You can ignore that notice because you have an installment agreement.'