Only a vet can tell you for sure, but my slimmest piggy weighed about 390g at 12 weeks and she did look and feel a bit bony. I would say a vet check is best if you are worried your piggy isnt gaining weight.
Do you weigh her at least weekly? If she was a tiny baby she might be gaining weight normally, but I think you should see a steady increase of anything from 10-50g every week though... and are you sure about her age? We think somebody lied or got confused about our little boar's age, he could be 4 weeks younger than we thought so he was classed as underweight when we first took him for a vet check...
Vets first, then a piggy weight diary I would suggest!