A.M.
It's definitely allergies. I went through the same thing with my son. I swore up and down my son had asthma. Went into the doctor's office, demanding a breathing treatment.... He was given a cough medicine with an antihistamine in it....long story short, it was allergies. What do you know?
I suggest doing some deep cleaning in her room.... underneath the bed, all the nooks and crannies, windows, bed linen, etc. Then I would suggest getting a plastic zippered mattress cover, and put her mattress in it. Seal the zipper area with duct tape, and get her a thin quilted mattress pad so that she won't sweat to death against the plastic cover. This, along with her bed linens need to be washed weekly. That should really help.
Hopefully the Singulair will work. Zyrtec works wonders for my son. Good luck. Allergies are a pain in the rump. But since I have terrible allergies, I've become pretty good at taking care of them.