A couple of years ago I had a piglet (only about 4 weeks old) who had meningitis. Unfortunately despite taking him to the vet as soon as I realised he was not well and my nursing him day and night I had to give in and have him put down. He did not regain his balance. I was told by another pig breeder of many years that it can be brought on by stress. In this case I think it was possibly because I moved Mum and litter. All the other piglets were fine.
First signs apparently are 'paddling' with the front feet while lying down.
Fingers crossed for your pig.
Di