I also thougt I was constipated but after the second salt-dosis I had to go to the toilet for x times.
It's nothing special that only few stones came out. Repeat with some more flushes and you will see other results.
Ultrasound only sees the stone(s) in the gallbladder, not the ones which are in your liver. I have a lot of trouble without any stone in the gallbladder....