Exactly^^^
The rear drum brakes on just about every vehicle made in the past 40 or so years is designed to adjust tighter when backing up and applying the brakes hard. If you've done this several times on new brakes, you've likely adjusted your brakes to the point where they are too tight.
Another possibility is that a spring let go or something, and getting the drum off will make that evident and the fix easy.
Find that rubber plug -- right near the brake cylinder -- take out the plug, and inside (have to sort of feel it, can't see much) you'll find a star wheel that is the adjuster. The deal with the adjuster is that it moves easy in one direction, and not at all in the other. The easy way makes the brakes tighter against the drum, so you need to spin it in the other direction. To do that, you need two tools -- a screwdriver or a brake spoon (a tool designed for this purpose, available at auto parts stores) and another smallish screwdriver or pick -- press the small screwdriver against a spring-loaded plate that only allows for one way movement, press it off the star wheel, then rotate the star wheel to loosen the shoes. Once they're loose, you should be able to pull the drum.
Note that sometimes drums stick on the axle flange. Best way is to pull and rock from side to side, and if force is needed, a sharp blow to the side of the drum works much better than trying to beat it off from the back, or prying it off, etc. Hit it as if you were going to hit the tread of a tire, versus the face and you'll have the right idea.
