Just out of curiosity, how old are you? And are you athletic? Overweight or physically fit? Was there any event that occurred with the onset of pain?
Was your ultrasound specifically looking for a groin hernia? It should be scanning the inguinal region / inguinal and femoral canal. It should also have you perform a “valsava maneuver” (basically you bearing down as if you were trying hard to defecate) as that can basically push the hernia through a hole and make it easier to see on an ultrasound. Typically this is with a dynamic ultrasound, meaning a little video is captured of any movement in the region. You need to make sure you had a groin ultrasound with valsava specifically looking for hernias, if you did not have the valsava maneuver it is less likely to be effective, sometimes patients are told to stand up during this process too because gravity can help as well.
Rarely, an MRI with valsava is necessary to show a small hernia. Dr Towfigh (who runs these forums) has done research on that topic.
Anyway, if you are athletic, it is possible you have a groin strain or pull, or even something like a “sports hernia” which is sort of an all-encompassing term that includes a variety of groin conditions that usually feature; a torn or detached muscle or tendon in the groin / pubic area, always with pain. More appropriate terms for these conditions include; athletic pubalgia, inguinal disruption, osteitis pubis, adductor tears or strains, etc.
It certainly wouldn’t be a bad idea to see an expert hernia surgeon who is able to accurately diagnose a groin hernia. Smaller hernias can be notoriously difficult to diagnose and many primary care doctors and even surgeons are not aware of them, let alone how to diagnose or find them. If you’d like to share your general location perhaps you are located near an expert hernia surgeon.
Good luck and keep us updated on your case.