Are you sure you're not pregnant?

I am so fed up having to explain to GPs and gynaecologists my justification for knowing, 100% for certain, that I am not pregnant. 

I get asked this question a lot due to my gynaecology history and on going treatments. It's a precaution that's necessary to take for some of the hormone treatments I need to take. 

It goes like this:

Doctor: "Is there any chance you could be pregnant?"

Me: "None at all" 

Doctor: "You sound pretty sure about that. How can you be sure? Not all contraceptives are 100% effective"

Me: "I know. For sure."

Doctor: "I don't understand how you can know."

Me: "I don't sleep with men. I'm a lesbian. I haven't done what is needed to be done to get pregnant."

Doctor: "oh."