It sounds like the drainage tube between your throat and inner ear has become blocked, causing air to become trapped in the chamber of the inner ear.
Increasing altitude results in lower pressure outside the chamber, with resultant expansion of air within. (A bit like those selophane wrappers around those crappy rolls you serve.)
As flying croc states, the good new is, that the problem usually sorts itself out, but don't fly until it does resolve. (Usually 2-3 weeks.) Avoid sudafed.