that is most likely from a feat. Either from a Nail Bomb or Sharpened Blade, which all assassins and also the Centurion have access to on level3. Then again you have heroes that can inflict bleed with attacks. So I cannot be sure if you encountered either the one or the other option or maybe both combined at the same time. Anyhow the feats are pretty strong, Not sure though how the result looks like with resistance or without. Anyhow note that the displayed "bleed" in your lifebar is not what you lose in terms of HP necessarily. Sometimes you can outwait bleed by backing off and then you will notice that you haven't lost the whole amount. Could be that the resistance kicks in here - again, not sure.
When you get hit however you will receive extra damage, so be careful