When you start flossing and you haven’t previously, there will be an excessive build-up of plaque between the teeth and around the gum line. Initial bleeding is not a problem; it just means that this excessive plaque is being removed and it may be a little stubborn because it has been there longer than it should have been, hence the bleeding.
If your gums bleed slightly for the first few days when you floss then do not panic and leave your flossing out for a day as the problem will then continue. If the bleeding is minor and something you can cope with then continue flossing daily and as your plaque levels decrease the bleeding will cease.
If, however, the bleeding concerns you, maybe it’s excessive, then do not hesitate to speak to your dentist about it.