Based on my LMP I was told that I was 8 weeks pregnant and during the scan we only saw a yolk sac w/o a fetal pole or HB, but I knew I ovulated way later than I normally do so I didn't worry as much. My doctor actually pushed up my dates 2 times because the baby was measuring bigger than normal.
A heartbeat is a great sign hun, but it could be possible you ovulated later than you thought. Good Luck!!