Interruptions

So basically I’m 17 and in my first serious relationship. I came out last year. So being with my girl is teaching me new wonderful things. She told me early o she would get herself off and I’ve been told growing up it’s wrong. However I got curious and actually found it to be enjoyable and a great stress relief. Now I still live at home and to make it worse I share a bedroom. The interruptions are getting ridiculous. But Idk how to stop them without telling my mum what I’m doing and why I want alone Time. If I told her the truth it would undoubtedly get me in trouble and a rant about how disgusting and wrong it is although I enjoy the times when I get to mainly when everyone is sleeping which can be ridiculous.