White Tip Tetras- Behavior Changed

[Kstawski]

So I have a 55 gallon, and I've had 6 white tip tetras in there for about 1.5 months. On Friday I added a piece of driftwood and a bristlenose pleco, and since then the tetras have been acting much different than usual. Usually they are very active, and often swim in the current caused by the filter, but since Friday they seem to gather behind a plant or in the corner and don't seem to swim around as much as usual. 
 
The pleco doesn't seem to be stressed. He dutifully clings to the glass and rocks, and doesn't pay too much attention to the tetras.
 
Is this normal? Or are they overly stressed by the new additions?  Thanks for any suggestions!