Why does your altered female ferret have an enlarged vulva?

There are several possible causes. The only person who can tell you for sure is a vet. Please take you ferret to a vet as soon as possible as a swollen vulva can be the sign of serious problems. I have put some possible causes below

The ferret is in heat - The female ferret has come into heat. She will need to be bred or given a special vaccination to bring her back out of heat. A female ferret will not come out of heat by herself and remaining in heat is very dangerous. Staying in heat can cause her to develop aplastic anaemia which can be fatal.

A partial spay - If the female was spayed at a very young age she may have what is called an incomplete or partial spay. This is rare but can occur when a female is spayed before she is fully grown.

Adrenal disease - If your ferret is also starting to lose fur (beginning at her tail and spreading towards her body) it is a sign that she may have adrenal disease. She will need medical treatment as soon as possible.

All of the above require immediate medical attention.