The Hornets were 2-1 up in the second half, though had to come behind from 1-0 down when Phil Jagielka headed the Blades in front while keeper Lenny Pidgeley was sent off for handling outside the box.
Jack Smith pulled things level and Heidar Helguson put the visitors in front but Jack Lester equalised with his penalty after Smith handled in the box.
Lewington said: "We could have won and we thought we'd won. It was an amazing game, but it was tough and it's amazing how incidents change games.
"They went 1-0 up, we had our keeper sent off and in the end we were a little bit unlucky but we kept going and showed absolutely fantastic spirit.
"I've no complaints about the sending-off, but as for the penalty - I've seen some given and some not, but over the season they even themselves out.
"I was really proud of the way they went about their work."