Nasal Congestion - can you become addicted to pseudoephedrine?

You didn't say if you were prescribed the Sudafed, or if it was over the counter. You also didn't say if it was combined with any other drug (many times it is combined with guafenisin, for example to control coughs.)

This is just a guess, you know, but what might be happening is this: you have been on the Sudafed for such a time, that your body has become adjusted to having it. When you go off of it, and your body readjusts to it, your blood vessels constrict and dilate, causing a headache (that could be very severe.)

This type of headache is very close to migraine. While no one really knows the origin or true cause of migraine, it is believed that blood vessel constriction and dilation is involved, and many of the migraine medications work on the relaxation of the blood vessels. Perhaps when you stop taking the pseudoephedrine for a while, this is the phenomenon that occurs in your head and around your neck as well.

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It certainly would be worth a visit to your doctor to check it out. Sinus headaches are similar in symptoms to migraine, without the presence of aura and light sensitivity, but not everyone experiences them the same. Either way, that this is occurring might mean a change in sinus meds is in order. There are many newer meds that won't have side effects like this that are on the market that your dr. can prescribe for you.