To be 13 months and 100 lbs is underweight, even for a growing female. My 10 months old boy is about 145 lbs. BUT it sounds like the people who owns her now, might not know a whole lot about dogs or mastiffs.
Since she is from a family, I would ask to get a vet health check. just to rule out the nasties. Most likely it is down to the feeding, and maybe worms.
I have to disagree with Senghe though, about OEM's being cautious. They should be outgoing and curious. I think the biggest hurdle when it comes to the disposition is that many people think they are so big, that they tend not to bring them places, not let them get used to things and other people and situations. She is still young though, so with patience and love I would think she can come around. Take her to the park, dont make attempts to have her talk to people or other dogs to start with, just let her look at them from distance, when she gets a bit braver, make it closer interactions, and maybe bring her to petsmart or any other store where they allow you to. Start with days and times of days when it is not too busy. Mastiffs are usually very kind hearted and loving, pretty much big lap dogs and bed hoggers lol.
Ask if they have the papers, that way you can (or if you need help I can help you) research the lines and see what is behind her, both physically and disposition wise.
The ear infection, is probably a yeast infection, they are not too uncommon on giant breeds well not on any breed really. There are great supplements to give to prohibit yeast and fungus infections both externally and internally. So I wouldnt worry too much about that either, a dose of antibiotic to start with, and then maintain with supplement and good cleaning.
About feeding, I have always had giant dogs, and more than one. I dont free feed, and as a precaution I never feed two dogs together if I am not with them, right there. Even the most darling dog, can act up when it comes to food, and it is an unnecessary risk to take.
Feel free to pm me if you want help with the line researchÂ
Marit