If they ever try to bring you somwhere private, oh you better be ready to ask for a steward. Theres a host of other things they could try to do or say to you, as in this matter, that is one of three things. That being, they cant do anything unethical, unsafe or against the law towards you. Here it sounds like a play at blackmail, and that is against the law. It is also unethical in a buisness platform. A future reference, you and that other steward should have figured, from the past, that is she wants to speak alone for any reason, she will pull this bully type advance towards you. Always have company, always, no matter the reason.
If you wouldve gotten it on tape, it wouldve priceless towards the claim. Moving on, once she uttered those words to you, the first action shouldve been stand up, exit the room and head for that other steward to make the claim as quickly as possible. Then return with that steward so, mostly you, and the other steward could re-hear what she said. As a person, first, one should know state laws as well as union contract details. What she did was totally against the law.
To me, she sounds like the type of person looking to get that gold star on her record. If its only you she doing this to, regardless of filing, missloads or anything else, it is harassment. Singling you out will always be harassment.
I'd say, threaten before you file against anything in the future. This has obviously put a target on you, because you come off as the type that will jump right to it before advocating a file. I've known a few people like you in the past, that would jump to paper in a second. All of them arnt here anymore.