Um, is "I'm not sick but I'm not well" appropriate?
I feel borderline sick -- not full-blown (no fever) but I don't feel up for going in to work today -- I don't think I could concentrate properly on the job. Argh. I've let the temp agency know, and my contact there will let the call center where I'm working know.
Better safe than sorry: I don't want to go from borderline sick to genuinely sick. Still, not how I'd expected to start my new shift.