Melatonin side effects?

After a sleep study I have been diagnosed with DSPD. 2 days ago I started to take melatonin after having it prescribed by a doctor. First time I took it, I took 1mg. I feel like I got super tired but every time just before falling asleep it felt like I wasn’t getting enough air and so ended up gasping for air every 5 minutes or so. Last night I took .6mg and it was just the same but less frequent. What could this be? I never took melatonin before in my life and since I’m currently under treatment for health anxiety, this isn’t doing me well….