
I would not contemplate breeding from your bitch until you know much more about the breed and lines within it. as you do not know of a stud then I am supposing that you have not been researching offspring of potential husbands by attending shows etc.
Breeding really is a very serious undertaking and should only be undertaken if you have a good knowledge about your bitches parents grandparents, and the sires. By this I mena know their strengths and weaknesses, their health temperament, and most importantly which of these traits were passed to their descendants.
If you do not have this in depth knowledge yourself then you should work with someone knowledgeable who has, preferably your bitch's breeder. If they are inexperienced, then I would proceed most cautiously and get the required knowledge first.
Bullmastiffs are a large breed, and I suspect have large litters. You will need to find competent loving homes for the puppies, and will be morally responsible for them for life. this may mean taking back pups you have bred at any time, and likely as unru7ly untrained hooligans, when their erstwhile owners have tired, or can't cope.
I suggest joining a breed club for a start. You will then be sent details of shows and educational events.
Lastly no REPUTABLE stud owner will welcome an enquiry from a bitch owner this close to her being due in season. Most people arange things with a Stud some months in advance. A lot of people have their breeding plans laid out several years in advance, lol!