A few years back I worked with one sow that did that and we decided to wean her at 5 weeks, as she produced good sized litters (10-11) that grew strong and quickly filled out and by 2-3 weeks was eating solids, but she ended up being very thin, if kept on too long which meant she couldn't be mated straight away after weaning. But if weaned early could, as she was still in a fit condition.
If your just breeding pigs to have piglets every now and then, then you can probably keep her on till 7-8 weeks and then just feed her back up again, and when she is back in good condition put her back to the boar. If your running a business/efficiently mating (not trying to offend anyone here) then weaning her early may well be the answer next time.