[shesinthebarn]

My filly has the same thing right now, and just today I started her on sulpha (essentially the same thing as you have). There is n ot a whole lot you can do other than what you are doing. If your horse is coughing even after treatment, you may want to look into and expectorant (like sputolysin), but you didn't mention coughing. It is most likely some kind of rhino virus you can't vax for. If she runs a temp, has laboured breathing etc, be sure to contact your vet.
As for the weight issue...have you ever thought that she may have ulcers?

 

[shesinthebarn]

well, the statistics are that 60% of pleasure horses and 90% of racehorses have ulcers. Those numbers are thought to be conservative now. If your horse is nervous or high strung, the likelyhood of ulcers go up. How is the maure? Is it ever loose or runny? The only way to definitively diagnose ulcers is through gastroscope, though. There are management meaures you can take if you do suspect ulcers.
Maybe try a low glycemic feed to help put on weight without adding excessive energy. Just a thought...