I would sometimes get a very small spot - only ever when wiping on the toilet - after sex when I was pg. The way it was explained to me is that there is increased blood flow to the area when you are pg and sometimes if you aren't properly lubricated then (and sometimes when you are) you can get small abrasions from the friction and with all the capilliaries close to the skin surface, they break easily and cause a tiny amount of bleeding. So in MOST cases it is nothing to worry about, but if accompanied by heavier bleeding and cramping, then you should get it checked out.