Hello Gazza
It could be that your sow is not pregnant at all - sometimes they will come back into season 21 days after being with the boar, and you have to mate them again as soon as possible.
If she is in pig, it will be 3 months, 3 weeks and 3 days before she gives birth - so note the week you got her, and project forward for a good idea of when she will be due.
Hope this helps - good luck with the pig breeding
Julie