What Causes Muffled Hearing?

Hi, i have seen my doctor this morning as I have muffled/impaired hearing on my right side..my husband noticed it over the last few months as I am missing what hes saying,talking louder etc my doctor says i have a healthy ear drum & there is no wax at all & im being referred for a hearing test..im just wondering what could be the cause of this muffled hearing if its healthy ear drum no perforations & no wax build up? I am 31 years old & have never suffered with any ear problems etc & dont have a cold..this is kind of been going on for months but i cant pinpoint exactly when it started.
thankyou Rose x