she will probably realize after half an hour that she can calm down and she can eat. And don’t worry about her not eating enough food because if she is in shape then missing one “meal” will not harm her. Horses are able to use their energy as needed, so she will be fine but she will probably end up eating anyways!
With my horses I either weigh out how much hay I give them before I give it to them or if it’s a square bale I give them eg. 4 cakes of hay with the weighing you can then weigh it after then work out how much they have eaten
Hope this helps