I assume you have had her vetted. A conversation about ulcers may be in order. I would recommend an equine dentist for the teeth check. I have a mare with TMJ and sometimes her jaw needs ‘adjusting’. I am usually alerted to this need when it takes her forever to finish her food and she has difficulty chewing up her carrot treats. Can you watch her eat to see if you notice anything else (other than the dropping feed)amiss? It does sound like a tooth issue. I would also recommend taking a fecal and then worming based on the findings and not just doing a rotation worming (but you may have already done that – I wasn’t sure as you stated she was ‘wormed’). Are there other signs of stress- pacing, chewing on boards, etc.? And I agree that it is best to contact the previous owners and 1. Talk about her feed and 2. See if this mare any of these issues before. Good luck. I am sure you are probably also stressing about this less than smooth transition for your mare.