I get similar pain in my lower right side after certain activities that involve my abdominal muscles. I went through all the tests that you have.....trans-vaginal ultrasound, colonoscopy, blood work for appendicitis, etc. Nothing showed up so my pcp sent me to a surgeon to see if he could find a hernia. He ordered a CT scan and it came back with the diagnosis of right-sided iliopsoas bursitis. This was back in 2019 and although I am still living with the pain when I do more work/exercise that involves my abdominal muscles, I can deal with the side effects by taking a pain reliever and resting with a heating pad on the area of the pain. There are other solutions to the pain that were offered to me, but, I am in my 70's and decided to opt out of them. Have you been examined for an inguinal hernia? Have you had a CT scan? Perhaps you might ask for that. I hope you find a diagnosis and then an answer for the relief of your pain. Don't give up looking.