Unfortunately, continued overuse of Afrin can cause chronic long-term
inflammation of the sinuses, a condition called "rhinitis
medicamentosa."  In fact, my ENT told me he's seen people who overused
Afrin and ended up with ulcers in their nasal passages.  Just like
people who snort cocaine regularly--the effect on the nasal passages is
similar.

If you're in that situation, you need to see a doctor and get either
prescription steroid nasal spray or maybe even oral steroids.  That's
the only way to knock down that chronic inflammation.

If that fails too, then it means you did indeed injure the mucosa of
your nasal passages with the Afrin.  In the absolutely worst case
scenario, you might now need surgery to scrape away that inflamed tissue.

Steven posted a fairly full answer.

A short course of oral steroid has a very good chance of doing the trick.
Your doctor (preferably but not absolutely essentially an ENT) can prescribe
that.