Foot off the Gas Pedal (Low Gas Milage)

Does anyone have any idea as to why my 2013 Prius is running at 1mpg when my foot is off the gas pedal? This is something that started occurring last night. I was driving through a pothole infested road when suddenly I hit a big one. As I continued to drive off I noticed an unfamiliar but slight buzzing noise as my car coasts onward. My mpg's have been destroyed since. I noticed that as I continue to drive my mileage seems to get better.