The Gallstones that you saw originally on the ultrasound were together in a bunch and appeared as one. Now that you have made room in the gallbladder by expelling soft cholesterol Gallstones there is room for the echo visible Gallstones to move about, they have split up and can been seen individually. Soft cholesterol gallstones do not appear on the ultrasound.
These echo visible stones commonly form in the deepest part of the gallbladder. There are the deposits of a gland that secretes chemicals into the bile but when the gallbladder is congested the chemicals hardened into calcified gallstones. They usually grow to about 10 mm in size. To get rid of them you will need to expel all the soft cholesterol stones and sludge that lays above Them. The gallbladder will then start filling and emptying properly again and you may expel the visible stones.
The gallbladder attack you had was probably a soft cholesterol stone travelling down the common bile duct. This is a good sign despite being incredibly painful.