I'll assume it's a zone of control issue.
1) Observe the red area around the city before attacking(enemies zone of control)
http://i.imgur.com/b6ZuzD0.jpg?1
2) Sacking doesn't use any additional movement(action points)
3) If you sacked Antioch and can't immediately move, it means you used all your movement getting to the city
4) The zone of control is gone the turn you sack the city
http://i.imgur.com/rfcjPNc.jpg?1
The zone of control returns on the next turn, providing the garrison has replenished somewhat(land/sea). If your within the zone of control you won't be able to move until you sack the city again
http://i.imgur.com/rRwG6CN.jpg?1
5) If you don't want to be stuck in the zone control the turn after sacking, you should make sure there's enough remaining movement to get clear of that area after sacking. One quarter remaining movement should be sufficient, maybe a little less. If your remaining movement is low, you'll have to use your best estimate and hope you have enough left over to get clear
http://i.imgur.com/Wg8IuLB.jpg?1