Are you trying to get her mated again in three weeks time? If yes, I would play it safe and sponge her to come into season just a few days after her three weeks - if you count today as day 0, then insert sponge on day 9, change over to 2nd sponge on day 16, remove sponge on day 23. She should then be in season two days later.
If you were to take her to get mated, she could probably travel in the back of a 4x4.... although ahemmm, that is probably not entirely legal. But at this time of year you shouldn't have too many problems with ventilation/over-heating.
Is this Amanda playing up? She could also just have re-absorbed the foetus, sometime in the first three weeks, hence coming back at irregular interval?