at around 6wks a sack and yolk sack should be visable, somtimes a fetal pole and heartbeat are also seen, I dont know what your doctor ment by mass usually they say a sack/yol sack or fetal pole was seen... to me a mass might indicate a molar pregnancy or similar, I hope your doctor has you in for a repeat u/s in a week or twos' time and hopefully things will become clearer then
You shold ask your ob more questions. I was bleeding at 6 weeks and through my first trimester due to hematoma. I had a healthy baby at 40 weeks nevertheless. At 6 weeks they may not see much on u/s (they could barely find a sack in the ER at 6.5 weeks for me) but just a few days later we saw the h/b. Baby develops rreally fast at this stage.