I was going to suggest bowel movement as a cause. I sometimes get blood if I've been a bit constipated. It will usually only be there on that wipe but occassionally i will have a little the next time i go.

I can tell by wiping and then holding a clean pieec of TP against my rectum for a few seconds. When i look sometimes there are a few blood spots. I occasionally get this while not preg but i think it's more common during preg cause of all that extra blood down there.