Actually it is common to bleed vaginally when passing bowels. The cause can be be from cysts on your ovaries or ovarian cancer or cervical cancer is what I was told. But that happened to me due to having large cysts on my ovaries that we're not cancerous and eventually ruptured and went away. They said its normal to have cysts on ovaries during pregnancy
If it's when you move your bowels are you sure that it's not rectal bleeding from piles? It's very uncommon to have vaginal bleeding to only occur when you move your bowels. If its when you wipe, look in a mirror or only wipe your anal region to see if the blood is coming from there, or if the blood is coming from your vagina, although as I said, I doubt very much it'll be vaginal bleeding if it only comes when you pass a bowel movement. If you have vaginal bleeding this far on in your pregnancy you should call your Dr x