
One of mine bled for 4 weeks and then stopped. Prolonged bleeding can just be them or can be a sign of cystic ovaries.
Also as this is her first season are you sure she hasn't had a split season, started then stopped due to the hormone levels being not just enough and then re started. This would not be unusual for a first timer, I have had that too.
If she is well I would do no more about it and watch to see the pattern of her next few seasons.
If she is to be a brood bitch and this continues it may be worth consulting a reproductive specialist, who may advise she is bred from sooner than later as it could be a sign of fertility problems. Though you would need to ask yourself the question could this be a problem that will be inherited.
Always worth finding out what patterns her female ancestors show, as they are often inherited.