I know that feeling. When mine died (and we still don't know what she died of) I was afraid to go and feed the other one in the mornings in case I found her dead in the ark like her sister.
The vet said that unless she was showing any symptoms then to send her off as planned and that, if there was anything wrong, the abattoir would find it. It was a great relief, that not only did they find nothing, but the butcher said she was a very good pig. Phew