Hi all, I’m a first time breeder and have done plenty of research in regards to breeding. My bitch was mated naturally on day 11 of her season, and then through AI on day 14 as she was still ovulating but the dog wouldn’t tie. We are now on day 19 but my bitch is still standing, flagging and presenting for males. Her swelling is very minimal now and bleeding has almost stopped and is dark in colour. Should I be concerned about her flagging behaviour continuing? Many thanks.

Not necessarily - I'd suggest she's just a flirt!  With dogs, who only 'stand' when ready to conceive, only one mating should be enough for it to be successful - 99.9%

Watch that discharge - I did have one who bled to the end of that season, after being mated, twice.   She had an uneventful pregnancy and produced 9 of what were probably the best puppies we ever had.   All the rest would fade to no discharge by around day 21.

Dark discharge just means the blood has oxidised, so is older blood than red blood.

Standing and flagging can often have no relation whatsoever to fertility and ovulation - some dogs will stand throughout, others only when they are ready.

Did you progesterone test?