Beginner with a sick axolotl

We are not experienced amphibian owners. We recently acquired our first axolotl, a golden axolotl, now about 3 months old. It is about 5 inches including tail. We have been feeding it blood worms from frozen cubes every few days. It developed a marked swelling around its throat. Its limbs look normal. After doing some research online, we treated it with furan 2 and did frequent water changes. It seemed marginally better, but not much. It's behavior seems pretty normal, its gills look normal, and it has a good appetite.

Our questions:
1) What could this be?
2) Should we try a different food? We bought bug bites and carnivore pellets to try an alternative. We would try earthworms, but we're in Massachusetts and the ground is frozen. We haven't found a retail source.
3) Is it worth it to try "fridgeing?" What does this actually do?

Thanks,
Elizabeth