About two cats who were best buds and now can't be in the same room together.?

Hershey "adopted" Jessie when I rescued 4 kittens from the warehouse at work. 3 of the kittens were given away but 2 returned, which left me with 5 cats... Hershey, Jessie, Callie, Casey, and a male cat, Thomas. Everyone is spayed/neutered. Last Sunday, something happened in the hallway, I couldn't tell what happened, and now Hershey and Jessie hiss and growl when they see each other. Hershey growls at the others, but relaxes when she realizes they aren't Jessie. If Jessie gets too close and Hershey really starts warning her, Jessie will advance on her and they will smack and spit at each other. I can't seem to get them over this and I don't want to lose either of them. Any suggestions???