Blood bits near placenta... (abit tmi sorry)

I went in for my 12week scan today.

The tech showed me that there is a bleed near the placenta (blood bits). Is this called subchorionic bleed? She told me to watch out for vaginal bleeding and to call straight away and not ignore it. She also said to not put pressure on that area by sitting up and pressing it down as it will cause the bleeding to come out.

Other than this bleeding everything else is fine.

Has anyone had this and what exactly is this? Why and how it happens? Also will it effect my pregnancy?

I just don't know what to think plus I do not have any knowledge of this.