"The way we were playing, I didn't think it was over at half-time," Williamson said after the 3-3 draw.
"We told the players the next goal was very important and that we had to get it. Unfortunately, we conceded. That's disappointing.
"Second half, we never played. Watford played all the football. They got at us and troubled us. Thankfully, we never conceded any more.
"We definitely had all the play in the first half and all the pressure but the manner in which we conceded the goals is obviously worrying."