Of course I am no ENT, but given it's been 6 weeks now since your Stapedectomy (I had mine exactly the same day
), I would think it is very unlikely that your cold has dislodged the piston. My understanding is that where such areas of the body a delicate and take their time to heal back fully to a optimal function, the body heals around the prosthesis very quickly and 6 weeks in only extreme or sudden pressure change or trauma is likely to dislodge it.
It's very likely the hearing loss and fullness is a result of pressure changes in your ear. Ive noted when you have a stapedectomy, you awareness of pressure changes in the ear is a little more elevated. My ears are popping and seemingly clogging up all the time without a cold. So anything like Sinus or nasal blockage to even inflammation will cause an effect. If you have a cold I would recommend keeping water out of the recently operated ear, as infections are common post a cold or virus.
The cold aside, I hope the hearing has been great, and you are recovering well.
Let me know how you are when the evil cold leaves you alone
Speaking of cold, it's freezing here in Sydney, coldest May week in many years. You will laugh when I say that cold here in Australia is when it gets below 10c overnight.