Most likely explanation is she did get in lamb and is aborting.
Another possibility, albeit slight, is that she is one of those ewe lambs which has a malformation of the rectum, where a fistula connects it to the vagina. Most usually this occurs where the anus has no opening, and the fistula allows the faeces to exit via the other passage. If she's had diarrhoea, and has this fistula, then it could be blood from the digestive system.
In a cow you'd ask did she cycle a couple of days ago and is it the post-bulling string, but I am not aware of this happening in sheep.