350 is small but as to being underweight you have to factor in how long she is. A picture would help. As kikary said rats are the better deal. I don't mess around with weighing food and just use the rule that the prey should be as big around as the widest part of the snake. I would probably put this girl on an every five days feeding schedule until she starts growing then back her off to a seven day cycle. It sounds to me that she was maintenance fed. Once she is on the proper sized food and a good cycle she will probably start growing fast.