2001 SL2 - low temp indicated and very little heat

I just got back from a vacation and my trusty 2001 SL2 has a new heating
problem. After starting it up and letting it idle maybe 5 minutes the temp
guage registers only a bit above "cold" . . . the first mark on the guage.
Usually it would be almost to the "normal" heat level . . . almost to the
1/4 mark. Also, very little heat is coming to the interior.

I figured I'd get a coolant flush done. They did a chemical clean and flush
to get rid of the sticky gunk that had shown up in the coolant resevoir.
All seemed fine when I let the new coolant circulate at idle. However,
after I left the shop, the temp started to go down again.

This is the first time since new that it's given me any problems with heat.
Is this a thermastat issue, or a CTS? Or, something else entirely???

Thanks.