Big Scare 😐

for about two days i had been having only brown spotting which i was told was okay. as the days went on the flow picked up and went on to bright red blood changing my pad twice in one day. i was again told that i was fine and not to worry. then i started bleeding really heavy and passing blood clots. i headed to ER where they drew blood and did an ultrasound. from my understanding i was about 6 weeks along but according to docs i was only about 4 weeks and the bleeding had only been implantation bleeding and i had nothing to worry about. gotta follow up in two days to make sure hcg levels are still going up. How is it possible that my LMP was about 6-7 weeks ago and i'm only 4-5 weeks along?