I'm so sorry about your Betta - he most definitely had something before you got him, and you should go complain to the store.

A bit of inside information on how they handle fish in the pet trade - it's not pretty.

I know someone who used to work in the trade, and he said that to keep the fish quiet and dormant so that they require the least amount of water during shipping, they dump cyanide into the water to shock the fish into a dormant state, drain out all but the minimum amount of water, and box them up so that more fish fit in a single package.

Some of them, alas, never come out of it upon reaching the store. This is considered "acceptable losses"...

Sick.