It'll usually return to normal of its own accord. Prednisone is just famous for wacking out your metabolism and making you gain weight in weird places on your body.
Hmm. Let's see...
It's easy for the body to develop a tolerance to antihistamines -- it happens a lot. Try switching antihistamines, even if you're on prescription antihistamines. I've had most excellent luck with 20mg cetrizine HCl (a double dose of OTC reactine, that you can get by prescription.) It's a great antihistamine for me, as it doesn't cross the blood-brain barrier and thusly has practically no sedative side-effects and won't (meaning shouldn't, I'm not that well versed on the LIST of drugs that interact with theodur) interfere with your asthma meds.
The important part about antihistamine pills is to take them ALL THE TIME. There are also other kinds of allergy shots that you MIGHT qualify for if it means you spend less time in the hospital when you're on them. When do you usually end up in the hospital? Is it with a cold, allergies, things like that?
The most important thing at this point (since you're already on so many anti-inflammatories) Is to keep the bronchospasm down. The more your airways spasm, the bigger the muscles get -- it's like weight training, except in this case, you don't WANT the increased muscle mass because it makes your airways smaller. If you need to start taking extra Serevent or possibly switching to Oxeze, (which works a little faster than Serevent... I'm on symbicort, you can get it since you're in canada, it's like advair, just different drugs.)
The important thing is to keep on top of it and keep pounding at it -- you sound like the kind of person who knows what they're doing and who's trusted by their dr. ... especially if you've been put on theodur.