This is an great answer (its great, should read the whole thing) about ancient egypt so not egypt at 100 BC, but the important thing to remember is that the idea of an ethnic identity as we see it is a modern idea (I can link a thread that talks about how it was developed if you want) so this principle likely still applied even at 100BC
the issue of whether someone is "an Egyptian" or "a Nubian" is a tricky one. According to Egyptian thought, if you dressed as an Egyptian, worshiped Egyptian gods, spoke Egyptian, and acted like an Egyptian, you WERE an Egyptian. For example, Hekanefer was an Egyptian official with a perfectly good Egyptian name and title, and his tomb and tomb goods show him as a typical native-born Egyptian. From the tomb of Huy, the viceroy of Kush, however, we know that Hekanefer (or his parents) was originally from Nubia.