Many piggies are afraid of unchartered open spaces. Try to put hides (from shoe boxes, draped old kitchen towels etc. in strategic places, about 1-2 ft apart and sprinkle lots of treats all over the place.
Expect that it may take a few times until Bombom will have the courage to really explore. You can also train her to come you the fence for a little treat. If you change the location, that helps to move her around.
Give her lots of encouragment and talk to her gently all the time.