Usually the milk doesn't come in til about 24 hours before they farrow. I had a similar thing with my Kune Kune, got a bit of milk one day then nothing - 3 days later she had 'proper' milk (squirting out if her udder rubbed on anything) and shortly after......piglets ! Again, with her the back end seemed to swell a bit, then be less swollen and all of a sudden it was VERY, VERY swollen. If she looks well, seems relaxed (eating and drinking okay) and doesn't have a temperature I would just keep an eye on her for the meantime.
It's probably just the equivalent of Braxton Hicks in women
Good luck !
Karen x