Causes of Nighttime Breathing Difficutly

I have woken up a few times in the past couple months gasping for breath. It feels like my airway is blocked. I gasp really hard and it wakes me up. The gasp is so loud that it makes my husband nervous. I have allergies and quite frequently my sinuses are clogged. So I can't breathe out of my nose, and have to sleep with my mouth open. I also have asthma but haven't had an attack in months. (I don't think these were asthma attacks.) Is my throat blocked because of post-nasal drip or could it be something more serious like sleep apnea?