One thought is that the swelling caused by asthma could be swelling up your breathing at night.
As person who has borderline sleep apnea I have noticed that the sleep apnea is less noticeable when I've lost some weight. At this time I personally would be doing myself a favor by losing 20-30 pounds. My question to you is 'Are you overweight?' If so, you may want to look into some diet changes so you can lose the weight. I personally found the reality show 'The Biggest Loser' very informative, however I've struggled with getting motivated to lose the weight. I personalloy do not believe asthma causes sleep apnea. Good luck in you search for better health.