Sounds like your provider is balance billing you.
Balance billing is the practice of billing an amount equal to the difference between the providers full charge and the sum of the reimbursement actually received from the insurance company together with any standard copayment, deductible or co-insurance.
It's common practice for providers to bill insurance companies full billed charges and adjust the charges after insurance has processed the claims at the contracted rate.
Laws vary from state to state but most states prohibit participating providers from balance billing patients for covered services. However, may states permit non-participating providers to balance bill.
I would your insurance company and explain that you are being balanced billed. They should the provider on your behalf with no problem.
Your only liability using an in-network provider should be your deductible, co-insurance and/or copay.
Best of luck to you!