I don't know what it is. I do know, though, that your doctor said your lungs were clear. He/she looked/listened with a highly professional ear. Their opinions do matter. If the sensation is not causing you anything but anxiety, try to consider this. Your body really, really, really got messed with!! You very recently had major with a capitol M surgery! You are made up of uncountable, miniscule parts -- many of which are fighting heroically to heal within you. You are going to feel some weird things! Nothing like this has ever happened to you, so how could you be familiar with all the feelings of recovery. Maybe it's just part and parcel of the whole journey of recovery.
Please continue to seek answers and confirmations; either with us or your doctors. And go with your instincts. If you really feel that something is amiss, insist that things be checked out as far as you want to take it. But also try to acknowledge that you just may not get any answers and the feelings may subside on their own in due time. Honor your body's willingness to branch out in new ways in order to heal.
Hang in there! Best wishes.
Marguerite