My two cats have stopped getting along

About 3.5 years ago, I bought two kittens. The shelter told me they were litter mates. They have been best friends every since. They play together, eat together, sleep together, and generally co-exist beautifully. They have the occasional spat, but mostly it's just playing. Something happened last Wednesday night, and they stopped getting along. Now, the female (the smaller of the two) hisses, growls, and yells everytime the male comes near her. I've separated them into two different rooms, so they can't see each other. They can still hear each other, but they weren't physically together. Everytime I try to reintroduce them to each other, the female gets hissy and growly and generally angry when the male comes near her. He then gets scared/annoyed and runs back into the bedroom. They have two separate litter boxes right now (her's is made out of an old shoebox, he has the real one) and separate food/water bowls. They have both been eating/using their litter box normally. What can I do? How do I reintroduce them to each other? I'm about to have construction done on my apt, and they were both supposed to move in with a friend of mine next week. Now I'm afraid that's not going to be able to happen!

Are they neutered/spayed? I'm wondering if something happened like a cat coming to the window outside that spooked the female kitty. I'm not sure what to tell you to do, however, except re-introduce them as if they didn't know each other.

Yes, they have both been fixed. I live on 18, so there were no cats in the window...