Thanks for using HCM.
Ruling out hernia
and diagnosing it on ultrasound
can be quite challenging and depends on expertise of the concerned doctor. Having said this, it can be possible that the time when you had the ultrasound, the hernia could have been small enough to be detected on the ultrasound. However you did mention that you notice a lump above the belly button which is a sign for hernia. Advise would be to revisit your doctor for repeat checkup.
Feel free for further queries.