Mirtazapine and asthma

My Mum has been on Mirtazapine for abour 6 weeks and even though they are helping depression she has noticed her asthma has been really bad. It started around the same time she started taking Mirtazapine and we are just wondering if it's connected or not. She's always short of breath and taking her inhalers all the time (more than she's needed to in a longtime). She's seeing the asthma nurse tonight (couldn't get an appointment with a Dr) but just wondering if anyone else had heard of breathing difficulties with Mirtazapine? Thanks