On tamoxifen for about 1 month. experiencing severe digestive problems, sharp pain when breathing in under left rib and nausea. no fever. is this pancreatitis?

Those symptoms are not consistent of pancreatitis but nausea, heart burn and rib pain (worse with taking a deep breath) are common side effects of tamoxifen . You can try antacid for digestive symptoms and Ibuprofen for rib or bone pain, as needed for overcoming these symptoms. They will all subside in a while. Pancreatitis may occur but it is uncommon.

I agree with Dr. Le that your symptoms don't suggest pancreatitis, but if you have a history of breast cancer, I'd be concerned about rib or base of left lung metastatic disease and would urge you to notify your oncologist of these symptoms.