My 5 year old daughter started on Singulair this past fall; she h...

My 5 year old daughter started on Singulair this past fall; she has what seems to be seasonal and/or dust mite allergies & gets a post nasal drip that makes her cough all nite long. We were thrilled w/how well it got rid of her symptoms but I am concerned and will be calling her doctor. First of all, is she going to have to take this every day for.........how long? Won't her body build up a tolerance? AND she's had some changes that when occuring just once, or isolated from each other, I would never think to blame the meds BUT - she has had repeated complaints of aching legs & feet, (I thought growing pains), almost nightly waking w/bad dreams, and itching all over her body. PLUS since she started kindergarden (when she started on Singulair) she's having all kinds of problems at school ie: not listening, not focusing & finishing her work; after a whole year of pre-K that went like a breeze & the tecaher loved her!