trying to measure my waist

ok i’m 23 about 120 lbs and 5’6. i tried measuring for low rise jeans and my hips are about 30-31 in, but whenever i go to compare to get a good jean size it says i’m a large on most websites. however im really small and when i go shopping i don’t fit jeans that are that large at the thrift i can fit size 2 jeans so why are my measurement like that