Why does France get excommunicated within the first few months of my 1444 game?
You can't be excommunicated if the papal state has positive opinion of you.
The papal controller can excommunicate any catholic nation that the papal state has a negative opinion of. Fortunately you can just improve relations with the papal state to avoid this as you can't be excommunicated if the papal state has positive opinion of you.
Or, you can drop Provence and ally the Pope. :)