I have a 12 year old mare that is acting sort of listless.?

She keeps laying down, but not rolling or anything. I bred her to a stallion back in June.Her and my gelding are basically inseparable but all of today she's been staying pretty far away from him. I tried giving her some hay pellets to see if she would eat but all she did was give the flehmen response with her upper lip a few times like she didn't know what it was. She won't eat anymore of our hay but she was eating some gr out in the field. I've been feeding her and the other horses round bales and the other two aren't acting strange like she is. Does anyone know what could be wrong with her?