yup, it was an old or improperly harvested eggplant.
even if it was picked and you bought it only days later, if it was allowed to ripen too much on the plant, it will turn bitter, and there's nothing you can really do to get rid of that bitterness.
i've noticed this in my own garden. even tho i may have just picked and eggplant, sliced, s&p'd, and oiled it to cook on the grill, i always taste a little raw piece.
sometimes they've gone too long and are bitter, so they get fed to the birds.
in nomine patri, et fili, et spiritus sancti.
Meh nom eh noh...doot dooooo do do do.