Toddler coughs only when he sleeps?? Asthma?

My toddler coughs when he sleeps only and sometimes it gets real bad with phlegm I told the doctor about it and she gave me allergy medicine for him to take at nights because she says it could be post nasal drip but it still doesn’t do anything. He’s not sick and he has this everyday only when he sleeps 😕 it’s been going on since the last time we got sick and this was 2 months ago!!