It's only July, a little more information would be needed to find out what is happening.
When you leave Verizon, per your customer agreement, you are billed thru the end of the current billing cycle. If your billing cycle ends on the 10th of June, for example, and you leave Verizon on the 12th you would be billed thru the 9th of July. Furthermore, your billing statement would not have been generated prior to your leaving on the 12th(usually takes ~5 days after your billing cycle ends for bill to be generated) so you WOULD have received a bill AFTER you had already left Verizon.
But like I said, with the information you have provided I don't know if this is the case in your situation and more information would be required.