Mine were treated in October - but then rather than waiting until March were wormed again on the 12 week mark......
My mare on this regime had a zero worm count from a poop count - but blood tests revealed tape worms, so she was given a double dose by the vet because of her colic.
If they have worm burden then worming can, as Wally says, bring on colic. Keep them in for 24 hours after worming so they aren't on the paddock.