Few things get someone with health anxiety fired up like blood where it shouldn't be! If it is blood then the fact that it's bright red is actually a good thing; dark would mean that it is old blood, and bright means that it's fresh, so it's probably just a small amount and caused by the act of having a BM. Your doctor said there were no fissures and no blood in the stool. The only things I can think of are that it isn't actually blood (maybe tomatoes or red peppers?) or that it only happens when you are having a BM so there is no trace of a problem at the doctor's office. If you fear the latter, then you might ask for a take-home stool test kit so that when you have a bright red BM you can bring that exact sample in for analysis. If there's still no hint of blood then it's caused by something else; most likely something you ate.