What do you think I should do?
Assuming you worked in an at-will employment state, there are very few things you can do.
If this bookstore is part of a chain, you could appeal to the HR department. You could explain your situation, what happened, and the circumstances involved. It's possible, if unlikely, that they could intervene on your behalf.
But while it's unfortunate, there is no law in your jurisdiction preventing your employer from firing you for any reason that isn't illegal (such as due to race, religion, etc), or for no reason at all. The reasons given may make no sense, be unfair, or be completely made up - it won't matter legally.
Most likely you'll just need to find your next job and put this one behind you.
