I had this though I wasn't pregnant at the time.... took myself off to the hospital! Apparently everyone has small pollops inside the anus, the doctor described them to me as a 'small bunch of grapes'. Haemorroids are these pollops which have popped out!! If you are constipated and/or straining then sometimes the pollops burst which is why you get the blood!! I also think the added pressure of pregnancy would cause that to happen too!! Just wait and see if you get anymore.... and if you are worried then definitely go see your doctor!!

Hope I helped!!