The woman came to the San Fernando Valley store Friday wearing a mask ... the manager tells TMZ. At some point, as she was shopping the mask disappeared from her face. She says she has a breathing problem and that's why she took the mask off, but her lungs were filled with anger as her voice billowed through the store.

Either way ... we're told employees approached the woman and asked her to cover her face, but she refused. At that point, she goes insane, suggesting this was some sort of conspiracy by Democrats.

She was ordered to leave the store -- which she did.