Hello~It may due to some pinched nerves in your lower back, if you are able, I would see a chiropractor, he/she will take some x-rays, study them and go over what can be done to help.
Ah, that's too bad. Is it sharp pain or dull? I think this article does a really nice job of talking about different things someone might be feeling when walking or standing with lower abdominal pain and what would cause it. https://www.healthline.com/health/pelvic-pain-when-walking
Lots of things can cause it from weak pelvic floor muscles leading to pelvic floor muscle pain (can be strengthened with exercises or see a physical therapist), dysfunction of the joints in front of your pelvic area coming from overuse or even vitamin deficiency, an inflamed sacrum, arthritis, a hernia (which was my personal first thought from what you wrote), etc.
An ultrasound wouldn't show all of these things. So, I'd see your doctor again to discuss!