I am using the fentanyl patch for lower abdo pain, currently undergoing tests with gynaecology. Am I ok to take dihydrocodeine when the pain is bad?

Concerned. As a doctor who uses narcotics often for palliative care, I am concerned about the use of narcotics in this case. The cause of the pain isn't known. Narcotics are known as masking agents because they mask the pain, they don't treat what's causing it. Whatever's hurting you continues unfettered. To answer your question, try not to mix narcotics.
You have . To discuss this with your doctor and make sure you aren't masking anything serious. That said, with his approval, it is probably okay to take additional medication for breakthrough pain.