Severe sinus infection...singulair?

back in april i was complaning about me thinking i was getting a chest cold. well to make a long story short i went to 3 different drs who all gave me different things it could be. allergies, asthma, or me just thinking its way worse then what it was. finally yester day i went to another dr complaning about my ears hurting and them feeling like they were draining. finally she said i had a lot of fluid on my ears, my glands in my nose were severly swollen, and that i have a severe sinus infection. my nasal drip dripped down into my chest which makes it feel like i have a chest cold. she prescribed me a nasal spray and singulair10mg. i looked up the singulair and what it was, and it said it was a asthma medicine. i dont understand if i have a sinus infection why she would prescribe me a asthma medicine??? any suggestions???