I would guess earwax buildup--I've had that happen many times, unfortunately.
It just needs flushed out. First time, I went to the doctor, but after that, I just did it myself.
You can buy earwax remover, like Debrox, or Murine, I believe. I usually use sweet oil, a few drops in the ear to soften the wax, then I flush it out with hydrogen peroxide, using a small bulb syringe.
Sometimes you have to do it several times a day, for a few days, but it always works for me!
Just be careful not to force the peroxide in really hard--gentle squeezes will usually do the job!