Why does Balaam say ה' אלוקי here and not in other places?

Did Balak's messengers see/hear the miracle of Balaam's donkey and the malach?

Why did Hashem convey His message through a donkey, one of the special creations on the first Erev Shabbos? Why did Hashem choose Balaam, who was evil?

"מי מנה עפר יעקב"