Yes, I suppose that makes sense. I have had cellulitis many times before (around 15 times), and usually I notice pain and/or swelling/redness in the foot before I start feeling groggy (if I get to feel groggy at all). But I do remember one previous occasion when I had flu symptoms, which receded, and only then were there any symptoms in the foot.
Anyway, I've got my flucloxacillin now, although I'm not sure why the GP has prescribed 14 days' supply; I usually get 7 days (4 x 500mg each day).