and a number of Capitol Ships ceded me by my Allies the Altarians
This may be a start. Those ships have maintenance costs, so depending on the complexity of them and the number of them, you may be hurting there.
The other thing to note, those new planets may be upgrading buildings right now. If a planet isn't currently having a military or social production going on, all the money that would have been spent on it goes into the economy instead. However, start building something and the running costs soar.
