3.5 months post op is way too early to worry , but like you say it's a paranoia shared by many . At that stage I was only just getting back to my pre-op satge and I had slight concerns at the way it was progressing . However , from month 4 on , things have really started to happen .
Back to your question , from what I've read , one reason why hairs may die is that the blood supply is insufficient to sustain the grafts placed . I've read of cases where the growth is good , but at some stage later on , for some reason the blood supply required to the transplanted area can cease . I don't think anyone really knows why this should happen , but if others do then please chip in . Having said that , it's not a common occurence as far as I know , so not worth stressing over , especially at your early stage .