thank you all for your kind words.
i can't imagine that it was anything to do with the mealworms--all my other geckos ate the same worms and all are fine (so far).
the black and bruised areas did make me wonder if there was some internal bleeding and/or organ rupturing. couldn't have been eggs as this was definitely a male.
in a way i am glad it was fast, at least that way he didn't have to suffer through too much. :/
i am going to call around and try to find a vet that will do a necropsy on him. at least that way i will have some answers and can perhaps share insight to others that may experience something similar.
rest in peace little one.