After looking very comfortable in the opening period, Swansea managed to throw away their lead in the second half and ended up deservedly losing the encounter.
The defeat sees the Swans' woeful home form continue, having won only four of the last 20 games at the Liberty Stadium.
"I was pleased with the first half generally. I thought we did well," Monk told Sky Sports.
"But for some reason the second half was a totally different game.
"We didn't come out and do what we had been doing well in the first half and pushing on from there.
"The second goal is always crucial, and obviously West Brom got it, and got their tails up.
"Our passing was sloppy and we didn't start the second half well at all."
The result sees the Swans dragged further into a relegation battle, with only four points separating the club and Sunderland in 18th.
"Players are going to have to know what it is to fight for this club and fight hard," Monk added.
"West Brom did that in the second half, and we are going to have to match that in the games to come."