A person is guilty of criminal trespass in the second degree if he knowingly enters or remains unlawfully in or upon premises of another. What this means is that if someone in the store told you that you could not return, or had to leave, and you stayed or went back to the store, that is trespassing. There are defenses available, so contact an attorney near Pullman if you have been charged.
If you were actually in the store the charge would be Criminal Trespass First Degree. If you were not in a "building" then it would be Criminal Trespass Second Degree (lesser charge). The most common way of being charged with Criminal Trespass in a store is being somewhere in the store that you should not be (i.e. Employees Only Section), or being told not to come back there again (being trespassed from the store at a prior time); or being in the store when it was closed. There are other ways, but the main point is that you were somewhere in the store that you knew you were not allowed to be, or at a time when you knew you were not allowed to be there.