13weeks pregnant vaginal bleeding after bm... not enough to need a panty liner... only bleed after bm and for sure vaginal... advice?

Wait it out: I am not sure why bowel movements should make you have uterine bleeding, unless you are heavily straining. For the time being I would wait the bleeding out and see what happens. If you are constipated which happens to some pregnant women, have your obstetrician prescribe a stool softener.

Watch closely: If you have more bleeding that requires tampons or pads.... go to an emergency room. Otherwise see your OB doctor for an ultrasound. This will show the condition of the preganacy and if there are concerns or issues you need to know about. An exam will help see if problems with your cervix. Hope all turns out well as it usually does.