Posh looked set for a huge win over their promotion rivals after Lewis had saved a penalty from Gary Roberts until Jack Hunt's last-minute strike deflected in off Ryan Bennett.
Lewis said: "I was waiting to pick the ball up when Ryan tried to clear it and it struck the bottom of his boot and went in.
"It's probably the most frustrating goal I've ever let in, coming so soon after I'd saved a penalty and after we looked like we had weathered the storm.
"We had played so well as well. I touched the ball about three times in the first half because we were so dominant.
"I was confident I'd save the penalty because he gave it away a bit where he was going to hit it. Having got that bit right it was then just a case of making sure I held on to the ball."
Source: PA
Source: PA