Abdominal pain only when stomach muscles are being used; what could it be?

Thank you for your question and I am sorry to hear of your stomach pain issues.  I think you are taking the best course of treatment by seeing your physician and trying to eliminate the myriad number of causes of abdominal pain. Without knowing a full history and exam it is impossible to offer definite advice, so maintain close follow up with your doctor as you work on diagnosing the cause.

Pain with increased abdominal pressure is most often due to some type of hernia. Keep in mind there are many different types of hernias. If your intraabdominal contents are pushed into a hernia defect this can cause pain.  Muscle diastases or separation typically causes a midline bulge, not pain.  Gallbladder problems typically cause pain when you eat fatty foods. The best study for this is an ultrasound or HIDA scan. Your surgeon is probably ordering a CT scan with Valsalva so that the study will view your abdomen while you are pressing down on it. This is a better way to visualize some hernias and muscle separation.  As surgeons, we generally don't like to operate on "pain "  without knowing the source of the pain. Is difficult to relieve pain if you don't know where it's coming from or why you have it.  

See a hernia specialist to rule out a hernia as the cause of your pain. Diastasis Recti is usually not painful. However, it is not uncommon to have a hernia within one. 

Remember that CT scans can be misread and there may be a hernia there.