Contract Still Valid?

[Doctorcisco]

I am currently the owner of an S corporation in IL that provides computer networking and connectivity services. I have a contract that was signed on July 1, 2008 as a business to business contract. Basically, they subcontract me out to third parties for specific projects. The contract states, "(Name of my business) shall invoice ****monthly for services performed. **** shall pay each invoice within 30 days from the date of receipt of invoice."

I invoiced, complete with documentation of hours spent, the only project they had hired me to complete at that time, on 8/3. Because the nature of my business is project related, I also invoiced for additional time spent in August at the completion of the project, 8/22. I understood that they would not pay me more than once a month but I wanted them to have my total figures before they presented the end product report to their end client.

However, today, having not received payment for the 8/3 invoice, I contacted them to find out when to expect it. At that time I received via email a "new" policy update on invoicing and payments. At no time prior to today was I ever informed there were any changes despite my supervisor asking if I had turned in all my hours twice previously. The new policy says that I must bill in whole month increments to be submitted after the last day of a month and that they will then pay 30 days after the invoice is received. So, basically, if I complete work in July 4, I can invoice on August 1 but won't get a check cut until September 1.

Does this violate my contract? Are they legally required to pay any invoices turned in prior to notification under the terms of the contract? If the payment clause is violated, is the whole contract null and void, including the non-compete clause?