Hi! I’m A 16-year-old Girl And I Am Not On

hi! I’m a 16-year-old girl and I am not on any birth control and I never took any other contraceptives. My menstrual cycle is now 11 days late and I have taken for pregnancy test and they’re all negative. I’ve never been to an OB/GYN before and my mom is aware that I am sexually active. I just don’t want to worry her unless I know something is wrong so I would like to know what else could cause me to completely miss a period & not be pregnant,