Angelothebear ... glad you posted your question.
Generally, most pure-bred dogs should not be bred, even if they are a champion and/or are health tested. A championship doesn't mean they should be bred; passing health testing doesn't mean the should be bred. It also doesn't mean they shouldn't be bred if they're not a champion and/or don't pass one test or another.
Specifically, you need to spend YEARS in a breed ... researching pedigrees and ailments, getting your hands on the finest (and worst!) dogs out there so you can begin to understand the nuances of what the Breed Standard is talking about. If you'd like to find more information about the breed standard, please let Google be your friend and type in "Mastiff Club of America" and from there you can find the breed standard.
Even more specifically, your bitch is too young to be bred. Breeding a girl isn't appropriate until 22months or so, at the earliest. It was only after winning at some impressive dog shows under judges who are also Mastiff breeders, and knowing our test results did we breed our girl, Jade ... at 28 months of age. And, when we bred her, we drove from Kansas City to Ohio and then to Pennsylvania, New Jersey and Virginia to look at and get our hands on the nicest stud dogs in the country so we could make a pick. We had stud owners mail us thumb drives with extended video footage of them moving to and away from the camera, and also in profile so that we could get a sense for the mechanics of their movement and skeletal construction. We researched the dogs in the pedigrees of these prospective studs, and considered the testing that had (and had not!) been done on these boys. Then, it was time to discuss contracts.
Breeding pure-bred dogs, if done correctly and in an effort to better the breed (as opposed to try to make some money, "calm your bitch down", give a puppy to a friend or family member, etc.), is not an easy task.
Please post pictures (front, rear, and side (while standing), plus closeups of her head from the front and side) and a pedigree of your girl if you're interested in some further help.
Otherwise, you may also find a AKC licensed Mastiff judge in your area who would give you a private assessment of your bitch.
Good luck to you ... and enjoy your little lady!
PS -- Do not breed unless you're committed to the possibility of your bitch dying ... many friends of mine have lost one (or more!) girl(s) due to complications during and immediately after carrying and whelping a litter.
