Well I don't know why, but I never saw this thread yesterday and I was on too
Anyhow, it could be a million and one things, also make sure to take notice of how she is sleeping, is she on one foot or two. One foot is a good sign, two feet shows signs of being lathargic and not able to hold balance. Does she puff up her feathers a lot or is she pretty alert and churpie.
It could be that your little budgie may just be going through a molt. (my one bird lost all of his tail feathers all at once) Check to see if new pin feathers are coming in. It could also be that the stuff you used for mites might have damaged the feather folicles and caused the feathers to fall out. You might want to try some vitamin e on the area, just a little tiny bit strait from the capsule, the area might be too dry and uncomfortable.
Unfortunately I am not sure as too anything more at the moment and will like Critter do some research and let you know what else could be the problem. Please make sure that you tell Neuroticah to move the budgie away from Romeo immediately. If she can put the birds in separate rooms. Birds catch things very easily from each other and I know she would be devistated if anything happened to Romeo.
I hope this has helped a little and please keep me posted!