Any one ever experienced this?

I did my first Sono yesterday at 8 weeks and 5days. Baby was doing great with a heartbeat of 168 but I was told they found some blood in me which my body will absorb if I don't bleed it out. I was also told not to freak out because it won't affect my pregnancy and the cause might be implantation bleeding that did not come out when baby implanted. Never heard of implantation bleeding Not coming out. Just this morning I noticed some old blood like bleeding very dark brown but not heavy and it only lasted about 3hours. I was just freaking out and still telling myself is normal.
Just wondering if any one else ever had this happened.