Unfortunately quite a few garden plants are poisonous, esp. Rhododendron and laurel families, but never heard of them causing bleeding from nose/mouth.
Searching keywords - sheep dead nose blood I found mentions of lung problems and aneurysm,
Holly is sited as a problem due to trauma, ie sharp leaves, enough to cut her inside?
Could she have got to any rat poison, that too was mentioned somewhere.
BTW, my sheep have lots of grass, but recently keep getting out, luckily not completely off my land.