Hello, Hector.
You receive a 100 points in the weekly siege ranking each time when you besiege a City.
The weekly ranking is updated every Monday at 7:00 AM UTC for all players around the world. At that time, all Siege points earned last week are reset to 0. Hence, when you start recalling troops from the besieged Cities and each time you get -100 Siege points.
There are 2 solutions:
1. Having recalled all your squads from the besieged Cities, first, you need to besiege some new Cities and earn new Siege points to reach 0 value. After that, all further Sieges will be bringing you points for the current Weekly Ranking.
2. Easier way is just to wait until the next Monday when your current negative value will be reset to 0 automatically.