I got a knee injury from playing hockey. Although it isn't yet serious enough for surgery - I twisted my knee, split the cap and as a result have a floating cap the doesn't sit central, which wears my cartilage away and could cause damage to my crucial ligament.. yeah its pretty painful and complicated. Anywayy.. when you say pop do you mean a cracking sound? If so it could be where you knee hasn't been mobile and moving freely due to swelling, a small amount or pressure builds up in the joint, when its released that sound occurs. Also it may be there has been minor adjustments in the gliding paths of the joint so the tendons could be snapping over tissue but there all harmless. When you start getting movement back into it and strengthening the surrounding muscles that may make a difference.. Hope it works out okay for you though.
May I ask why and what surgery you had?