I would have thought it could well be the baby kicking. I have 2 pregnant mares at 8 months and 8.5 months, a foal at (8 months ish)240 days would be about the same size as a lamb 45lb and grows to about 70-80lb in the next 30 days, so not that small anymore, if you look you may see the movement under her tummy. If you can see it she can certainly feel it! On one of my mares I have seen the baby kicking quite obviously although she is very laid back about it! The other mare will turn around and glare at her belly. lol. The first mare has been through it all before the second is a maiden mare. Lunging the mare may have caused the baby to agitate a little, as she returned to normal quickly I wouldn't worry, it may well have been the baby having a good kick.
However, if you see her doing it a lot it could indicate a problem with the pregnany or colic and you should clear it with your vet.