Sinus infections, as far as I know as a non-doctor, can cause loss of sense of smell. I don't think allergies would, unless they caused a lot of congestion and maybe a bug got in on top and caused an infection.
If you have throat drainage, I'd guess a chronic sinus infection. If it's gotten in the biofilm it might just hang around and not cause acute problems except occasionally.
Did you see an ENT? He could order a head scan to see if there's a blocked sinus. Mostly they just give you antibiotics for ear infection and send you on your way.