Mikey14: It is conceivable that the symptoms you are feeling are attributable to the stomach being squeezed at the hiatus as you suspect during periods of increased abdominal pressure, but I can’t give a strong opinion about what you are individually experiencing without examining you, watching you do a sit up, and having more data, such as the results of a CT scan and/or EGD. Other causes could contribute to your discomfort, such as diastasis recti, Chilaiditi syndrome, intraperitoneal adhesions, etc.
I would advise continuing to pursue further evaluation, and if your current care provider is not confident in a diagnosis then request referral for additional consultation. If EGD to assess the extent of the hiatal hernia has already been done (symptomatic HHs and even asymptomatic hernias if they are large – more than 3-4 inches – should almost always be considered for surgical repair), and your doc doesn’t currently recommend repair, then further testing with esophageal and gastric manometry may give you more information about pressure gradients in the upper GI tract. If this testing were done while you were actively attempting to brace your core, it could provide good insight about the location of any high pressure that is contributing to your symptoms. If you have not yet been assessed by a general surgeon or a gastroenterologist, ask your PCP for a referral.