Constant Increase in Internet Billing

Over the last 6 months, my internet bill has been out of control. The charges don't make any sense. We have an unlimited plan but there are overage usage charges applied to almost every bill. My bill ranges between $190-$360 per month. This is a drastic change in billing. I called customer service a few months ago and ensured me that we are on an unlimited plan but just last month (Feb-Mar) I was charged $238.73. I can't even open the statement to see the details. This is becoming extremely frustrating, and I need to understand why my bill is not consistent each month.