What are the temps in the enclosure? Basking spot and cool side. What kind of UVB bulb (compact/coil, straight tube, mercury vapor)?
If it's too cold, if can certainly slow them down. Too hot can cause problems, as well, if the enclosure isn't big enough for them to get away from the basking area to cool off. As well, there are certain UVB bulbs (especially compact/coil) that have been known to cause eye irritation, which, in turn, can cause other problems, including lethargy...
I'd also recommend a fecal exam from the vet to test for parasites, since they are rescues.
Hopefully it's nothing serious!