I agree with Joan, but its possible it could be dust mites. They have special covers for mattresses and pillows that certainly have helped me. I went to an allergist to find out that I am allergic to cats, dust mites, mold, mildow, yadada -- at the end of the visit I asked why I could be around certain things that I was highly allergic to, but had no reaction; and other things that didn't even show up on his chart that drove me absolutely crazy. The dr. smiled and shrugged. I left with the uncomfortable feeling that its all voodoo science, but what the heck, we don't have any better alternatives than getting tested at this point.
Ironically, I can now go to bed sightly asthmatic and wake up feeling better...
Best of luck, asthma really does suck, I know exactly what you're going through!