I saw this also and asked on the Pax forum.
On 2/4, I updated Moon locations (all now at 100%), and paid several days of bills at previously recorded levels. Dropped Resource trade and later in day accepted same resources as trade from different nation.
Paid bills normally on 2/5. 2/6 and after, total bills increased by +9.8%. New Infra-only bill greater than previous total bill.
No events, no new or expired Infra upkeep modifying senate proposals. Crime Index remained above 500 throughout.
Basically a 10.8% increase in Infra only bill on 2/6 with no apparent cause.
I know of one other nation with similar increase on that specific day and remaining since.
Has no one else experienced this? Maybe I was getting a good deal for awhile?
Edit: Looked at my Activity Log. Bills were normal on 2/5 and increased on 2/6.