A first-half header from French defender Mathieu Baudry and a Kevin Lisbie penalty early in the second half gave the Londoners the three points.
Turner was disappointed with how his side switched off to allow Baudry to score and was also not pleased with the rash challenge by David McAllister on Romain Vincelot which led to Lisbie's spot-kick.
"We've hit a flat spot and three consecutive defeats is the last thing we wanted at this stage of the season with so much at stake," said Turner.
"You can feel a little bit of tension about us and we've paid a heavy price for that.
"Our defending for the first goal from a set piece was quite poor. We didn't get ourselves set up quickly enough.
"And what kills you dead is the goal five minutes after half time. You can't go into tackles like that. David knows it, he apologised to the players, but it doesn't do us any good now.
"We conceded right after half time at Walsall last week as well and it kills off the inspiration to try and start afresh in the second half.
"We had efforts at goal, one or two trickled wide, but we weren't really convincing."
The Shrews sit just four points clear of the relegation zone and face testing away games at MK Dons and Crewe this week.