Tire changing/Wheel removal

[Catrin]

A year ago I practiced changing a tire on my Trek - that was quite an ordeal. So much of an ordeal that it has taken me until today to practice the same thing on my LHT - a far easier experience

The problem was that I wanted to practice on my rear tire, but I couldn't get the wheel off the bike

I released the rear brake, the quick release, and the black "nut". Before I did any of this I shifted the rear cassette down to the smallest gear and made certain the chain was on the middle ring - and no matter how much I played with the quick release I could NOT get the wheel off the bike!!! I DID move the rear derailleur out of the way while trying to remove the wheel. I wound up practicing on the front tire.

Any ideas on what might have been the problem? The wheel didn't even feel loose, and I almost unscrewed the black thing in the quick release off! And now I can't get my rear brakes to line up which leads me to believe that the wheel did shift and isn't in straight any more - though it looks fine. I am going to set it aside for tonight and revisit this tomorrow...