i would be asking your doc for a further dating scan later (another 2 weeks or so) just to see how things are traveling - you might just have a small bubba as it sounds healthy with that heart rate! u.s usually have an error window of about one week at that early. with your pcos, it's very possible you o'd late - although 3 weeks would be very unusual if you got an early BFP

speak to the doc babe - and if you need reassurance, ask for another scan...

ETA - i got BFP at what would have been, by scan dates. 3w1d, so it's possible hun!