Tussionex is the strongest cough medicine you can get, and it has quite a street value due to the opiate content. It's pretty stout stuff along with the antihistamine. So yeah, you've got a little opiate withdrawal along with the antihistamine rebound that el_em mentioned. No fun for a few days and you probably won't sleep well for a few nights, but you'll get through it.
Most docs don't prescribe tussionex for more than a short-term cough. If your doc has been prescribing it more than 6 or 8 weeks at most, you need to find another doctor who can treat the underlying condition that is causing the cough. Tussionex only covers up the symptoms, and as you know now, it cause unexpected trouble in the form of physical dependency.
I get the same thing when I stop taking my Afrin. It's not 'addiction' in the classic sense of the word... it's more like your body has become "med dependent". If you're not getting high from the stuff, then I'd say that's what you've got going on.
My stuffy-head and flu'like symptoms is my body telling me "Take some more Afrin!", but if I tough it out, it passes within 12 hours and I can be Afrin-free shortly thereafter.
I know this isn't your exact situation, as yours is Tussionex and mine's Afrin (nasal spray), but you get the idea hopefully.
Take care and good luck!
LMNO
How long have you been taking it? Your body probably has become dependent on it and that is why you are having the withdrawal symptoms. It doesn't take all that long for our bodies to get use to something. Are you thinking about putting it down for good?