How do i know if my rabbit has a tummy ache?

shes been quiet and she usually is for like 10 mins runs around explores and lies down but today she's been lying down since 9 and its 12 now and she ate like half a stick of celery and 2 slices of apples i know i spoil her and shouldn't i just need to know how to tell if she has a stomach ache PLEASE HELP MY BUNNY!!!!!!!!